(skip to the AH section, or the rough timeline, if you want just the meat)
Background
At the Battle of Soissons in AD 486, the Franks under the leadership of Clovis I defeated the last remnant of the Roman Empire in Gaul (known by various names, most popularly the Kingdom or Domain of Soissons), led by Syagrius (one of history's many 'last of the Romans'). Clovis's victory secured Frankish dominion over Gaul and led, of course, eventually to the creation of France.
It also extinguished the last remnant of the Western Roman Empire and sealed the fate of Western Europe as it fell into barbarism. However, this is not for the simple reason of 'Romans = civilized, Germans = barbarians'.
First, we must consider that the major Germanic tribes were not all the same in their culture, sophistication, and expertise; rather, they existed on a continuum of various axes of civilization. For example, among the most civilized of the tribes were the two Gothic tribes. Having has the longest contact with the Empire, they had great respect for Roman society and its institutions (despite consistent mistreatment by the same Romans) and they generally did their best to protect the Empire (though they ultimately, of course, failed). Somewhat less civilized were the Burgundians, who, while still fairly savage, maintained a strong culture of poetry. Least civilized were Vandals and Langobards (Lombards), who contributed almost nothing to European culture (other than some place names) and generally despised the Romans. (the Saxons were almost as uncivilized, but managed to overcome that).
Where do the Franks fall on that continuum? On the one hand, they were one of the more savage tribes and similarly despised the Romans (Frankish Salic law opened with a prelude explaining their triumph over the 'heavy yoke of the Romans). On the other hand, they were skilled agriculturalists, unlike many of the other itinerant tribes. Lastly, they were the first of all the Germanic tribes to convert to Latin Catholic Christianity, as opposed to Arian Christianity, allying them with both the Pope in Rome and the Byzantine Emperor in Constantinople (the Burgundians and Visigoths followed suit, which might have mattered if their kingdoms lasted).
So, of all the Germanic tribes, it was, ultimately, two of the more barbaric, the Franks and Saxons, the triumphed. The Vandals fell to the Byzantines, the Ostrogoths fell to the Byzantines and Lombards, who ultimately fell to the Franks much later, the Visigoths to the Arabs and Franks, and the Burgundians to the Franks.
The Frank and Saxons, for all their success, carried with them a fatal weakness, in terms of social stability: division of the territory (gavelkind or salic patrimony). The Merovingian and Carolingian Frankish rulers repeatedly divided their realm with every generation, as they considered their realm to be their personal property to be divided among their sons. And Anglo-Saxon Britain was nothing if not divided. A fragmented society such as this is not a stable environment
There were other reasons for such division, such as the inherent pragmatism of relying on the nearby lord for defense against whomever was coming down the road, and the tendency of officials to make their positions hereditary and landed.
Of course, the Frankish kings attempted to solidify their kingdom, and Charlemagne made many efforts in such matters, but, as we all know, his empire was soon divided as was traditional among the Franks.
Alternate History
So, lets say that Syagrius wins at the battle of Soissons and Clovis is killed. Most of the other Frankish rulers, as well as his sons, do not compare to Clovis in ability or foresight (one of the reasons that ultimately led to his dynasty's downfall), thus freeing Soissons from threats on that front and giving them some breathing room.
They might even be able to score some successes against the Franks or other barbarians in Gaul. The Franks will be fairly well divided and under sub-par leadership, making them an attractive target (as well as the greatest long term threat). The Burgundians also might be a good avenue for expansion: in addition to providing a contiguous link with Italy and the Mediterranean, they also are somewhat inclined towards Catholic Christianity (their last historical king was a canonized martyr).
Soissons thus occupies a position somewhat similar to Ostrogothic Italy: an effectively independent realm that nominally is ruling on behalf of the Emperor in Constantinople and seeking to maintain somewhat close ties. There are, of course, differences: Italy is richer and more urban, while Soissons is much more rural. On the other hand, Italy is ruled by Gothic Arians, while there is no cultural or religious difference between the rulers and ruled of Soissons. Geographically and politically, Soissons effectively takes the place of early Merovingian France, with a few key differences: the leadership is composed of more sophisticated Gallo-Romans, and they regard the ruler and the office as separate, avoiding the issue of the division of territory, avoiding the disintegration of the state.
Ostrogothic Italy is important for the development of Gallo-Roman Soissons for another reason: Theodoric's actions ultimately will have great influence on Soissons' fate. Historically, he attempted to unite as much of the Western Roman Empire as possible, arousing the ire of the Emperor in Constantinople, ultimately leading to the fall of Gothic Italy and all the problems associated with that (deterioration of urban Italy, the Lombard invasion, the economic costs in the Eastern Empire and the alienation of the heretics in Syria and Egypt). If Theodoric attempts such a policy, he may or may not seek close ties with Soissons. If he doesn't attempt such a policy, he'll likely seek close ties with Soissons, as another remnant of Roman civilization. If Theodoric tries to go it alone, then Soissons becomes the obvious foil for his ambitions, thus leading to an alliance with Constantinople.
Either way, we're basically replacing Frankish Gaul with a Gallo-Roman Gaul. We'll say that the Domain grows slower than the Frankish kingdom, but, due to its political institutions, is more stable and secure. Meanwhile, this stability will assist in the cultural growth of Gaul. When I speak of cultural growth, I'm pretty much referring to churches and monasteries, the main outposts of culture in early Dark Age Europe, particularly in the instances where the clergy and monks are literate. Its possible that a Gallo-Roman Gaul would have a wider cultural base, with the possibility of a literate aristocracy, but that isn't assured; we will simply assume that the aristocracy of this Gaul would be more literate than that of our history's Frankish Gaul was, and that the stability of the realm allows the clergy to focus on other matters, rather than constantly trying to civilize their rulers (the main task of the Latin Church in the Dark ages). Its this cultural base that is necessary to maintain the government.
So, as the Domain of Gallia, as we'll call it now, expands over the next few centuries, it forms a semi-civilized bulwark, incorporating the various tribes in Gaul as best as they can. The various members of the Frankish and Burgundian 'aristocracy' intermarry with that of the Gallo-Romans, as happened historically (though likely, with the Gallo-Romans ascendant, many of the marriages will be inverted; with Gallo-Roman sons marrying Germanic daughters), as the Church works to convert them to Latin Christianity.
The cultural strength of Gallia will prove a boon to Britain, as well. Without going into a long tangent, the cliffnotes version of the history of the Anglo-Saxon church is this: British clergy fled the Anglo-Saxons for Ireland, and converted the local. The Irish clergy came back and converted the Anglo-Saxons. The Anglo-Saxons then went and sent all their best minds and advisors to strengthen the Frankish monarchy (and convert the Frisians and Germans). The classic Dark Age standard of a Benedictine monk advising a king or lord often involved an Anglo-Saxon monk advising a Frankish lord. Then, the Vikings came and ruined everything.
With Gallia having a native source of educated clergy and aristocracy, the Anglo-Saxon clergy can focus their efforts locally, possibly even uniting England before the Vikings get there, so they can provide a unified defense (they might even get some assistance from Gallia). Or, possibly, they send their missionaries further afield, converting tribes beyond just the Germans.
Gallia will likely be closely allied with the Papacy, regardless of what happens with Ostrogothic Italy; unless the Ostrogoths manage to convert from Arianism, which would complicate things (but in a good way). This could lead to an analogue to Charlemagne's Coronation as Emperor. Except, now, instead of a semi-literate barbarian, it would likely be a literate Roman. Perhaps even with Byzantine approval (perhaps in exchange for military assistance against Persia/Arabs). Of course, with a Western Roman Emperor, Ostrogothic Italy, Visigothic Spain, and Anglo-Saxon Britain would technically fall under his domain, though they'd probably be as subservient to him as they were to Eastern Roman Emperor, historically.
Rough (best case) Timeline (just for fun)
Late 5th century:
Gallo-Roman forces prevail over the Franks, gutting the capable leadership of the tribe. A few of the smaller Frankish lords along the border submit to the Gallo-Romans. An alliance with the Burgundians to the south, who convert to Latin Catholicism from Arianism.
Early 6th century:
The Gallo-Romans push further into Frankish lands, taking everything north of the Meuse river. Several smaller tribes are re-settled further within Gallic territory, particularly on marginal land. The Gallo-Romans and Burgundians invade Visigothic Gaul, with most of Aquitania going to Gallia and Provence going to the Burgundians.
The Ostrogoths avoid becoming entangled in the war, to mollify the Roman Emperor in Constantinople. They instead ally with the Romans in their invasion of Vandal Africa, taking the rest of Sicily, as well as Corsica, Sardinia, and the Balaerics.
The Papacy remains firmly aligned with the Ostrogoths, who support the Pope, without converting. The Emperors in Constantinople maintain a general policy of religious tolerance, causing some friction with the Papacy, as they avoid suppressing the heretics in Syria and Egypt.
Late 6th century:
The last Frankish enclaves west of the Rhine fall to the Gallo-Romans, while the eastern tribes are conquered by the Continental Saxons. The Burgundian kingdom collapses into civil war and the Gallo-Romans take advantage of the power vacuum, incorporating most of their territory.
The Ostrogoths repel repeated incursions by the Lombards into Italy. To bolster their support among the population, the leadership converts to Catholicism, thus encouraging the Papacy to assist in the war. However, they are unable to hold onto Dalmatia, which falls to the Lombards. Meanwhile, the Saxons conquer the Frisians and Thuringians. The Visigoths, the only remaining Arian power, also converts to avoid diplomatic isolation.
The Eastern Roman Empire secures most of Armenia from the Sassanid Persians, and allying with the smaller states in the Caucasus.
Early 7th century:
The Gallo-Romans take the remainder of Burgundian and Visigothic territory in Gaul, solidifying their borders along the Pyrennes and Rhine. The conquest of the western Alemanni and the subjugation of Armorica (Brittany) completes the restoration of Roman Gallia. Gallia is the least urbanized of the three major post-Roman states in the west, but the only one that is governed by Romans. The various Germanic federates living with the territory are generally allowed to live under their own laws.
The Avars conquer a vast swath of land from the eastern Black Sea coast, along the Danube, and to the Saxon border. The Lombards fall under their rule intermittently, though the Avars have difficulty holding onto the region.
The Eastern Roman Empire is invaded by Sassanid Persia, seeking to regain Armenia. Due to poor leadership, the Romans face several setbacks and soon Persian armies are marching in Anatolia and reach Constantinople. Seeking assistance, the Romans ally with the Arabs, unified under their new religion of Islam. The alliance bears fruit quickly, as the Sassanids are crushed under combined forces, and are annexed by the Muslim Caliphate. However, relations quickly sour between the Arabs and the Roman Empire, and war breaks out between them, with parts of Syria and Judea quickly falling to the invaders.
Late 7th century:
The Eastern Roman Empire is hard pressed by the Arabs, who invade Egypt. At the same time, as frontier garrisons in North Africa are pulled to face the invasion, Berber tribes begin to pour over the border, and the Avars threaten the Danube border.
Meanwhile, Hispania, Gallia, and Italia are fairly peaceful and stable. The only major threat to Gallia is the Saxons, who are more focused on maintaining their border with the Avars (though they intermittently send raiding parties over the Rhine). The Anglo-Saxon kingdoms have congealed into only three kingdoms.
Seeking assistance from the cash-poor but soldier-rich west, the Eastern Roman Emperor sends envoys to the Pope in Rome, the Magister of Gallia, and the two Gothic kings for assistance. With the Pope acting as a mediator (in return for some theological concessions regarding the Pope's authority), the Emperor elevates the Magister to the rank of Emperor. The Visigoths and Ostrogoths officially fall under the new Western Emperor, though they maintain the same level of independence they previously held. Spurred on by the Western Emperor and Pope, the Gothic kingdoms join in the alliance.
Gallic forces are divided in two; half the army is sent with the Visigoths to secure North Africa, while the other half attacks the Avars in Lombard Dalmatia, with the Ostrogoths. The Lombards, unhappy with Avar dominion, ally with the Western Emperor and their kingdom is recognized as equal to the two Gothic kingdoms, contingent of their conversion to Catholicism. The Berbers, meanwhile, are offered a similar deal; though they do not hold as much power in North Africa, allying with them allows for the Western Emperor to simultaneously focus the military efforts on the Arabs as well as take North Africa from the Eastern Emperor.
Early 8th Century
The last remnants of the Arab invasion of the Eastern Roman Empire is removed, nominally restoring the entirety of the Roman Empire, minus Britain. The Arabs turn east, focusing their energies on India. The Western Empire, while theoretically containing almost as much territory as it ever did, is, in reality, confined to Gallia. The Western Emperors make no effort to expand their power beyond that, since they do not have the manpower or economic base to take either Italia or Hispania or even enforce anything beyond the nominal allegiance of either kingdom (or Lombardia or Africa).
The sole effective unifying force of the Western Empire is the Latin Catholic Church. Tied together by the clergy and Benedictine monks, the Church works to maintain the alliance between the kingdoms of the West, maintain their independence from the East, and, occasionally, strengthen the power of the Papacy (the clergy don't always feel more loyalty to Rome than to the local ruler).
Comparison to OTL: For the typical peasant, life is pretty much the same. The ruling class and clergy are more literate, which would ameliorate some of the worst parts of living as a dark age peasant; its better to live under someone who has some knowledge than just the best local warlord. However, life is still very much rural, compared to antiquity (with the exception of the Mediterranean coastal regions). On a political level, the component kingdoms of the west are, for the moment, more stable than their counterparts, and Italy is in much better shape. Western Germany is partially unified under the Saxons, who are taking the place of the eastern Franks of our history. Britain's doing about as well as in our history, and will have about a century to get its act together. The Eastern Empire is in very good shape, when compared to our history; Syria and Egypt have been damaged, but are still in Roman hands and are, comparatively, loyal as the Eastern Orthodox church and the Emperor have been more accommodating. The Western Church, however, will likely not approve of such accommodation.
The main remaining challenge to the west is the integration of the Germanic nobility (at least, outside of Gaul) and the combination of Germanic law and Roman law. This was not an insurmountable task for even our history, and the greatest Dark Age rulers made significant inroads into such matters; the rulers of this Dim Age should but up to the task. With any luck, they might be able to come up with a code of law that combines the best features of Roman and Germanic law.
Anyway, I'm out of steam. Enjoy the ride, pick things apart, argue and carouse.
Background
At the Battle of Soissons in AD 486, the Franks under the leadership of Clovis I defeated the last remnant of the Roman Empire in Gaul (known by various names, most popularly the Kingdom or Domain of Soissons), led by Syagrius (one of history's many 'last of the Romans'). Clovis's victory secured Frankish dominion over Gaul and led, of course, eventually to the creation of France.
It also extinguished the last remnant of the Western Roman Empire and sealed the fate of Western Europe as it fell into barbarism. However, this is not for the simple reason of 'Romans = civilized, Germans = barbarians'.
First, we must consider that the major Germanic tribes were not all the same in their culture, sophistication, and expertise; rather, they existed on a continuum of various axes of civilization. For example, among the most civilized of the tribes were the two Gothic tribes. Having has the longest contact with the Empire, they had great respect for Roman society and its institutions (despite consistent mistreatment by the same Romans) and they generally did their best to protect the Empire (though they ultimately, of course, failed). Somewhat less civilized were the Burgundians, who, while still fairly savage, maintained a strong culture of poetry. Least civilized were Vandals and Langobards (Lombards), who contributed almost nothing to European culture (other than some place names) and generally despised the Romans. (the Saxons were almost as uncivilized, but managed to overcome that).
Where do the Franks fall on that continuum? On the one hand, they were one of the more savage tribes and similarly despised the Romans (Frankish Salic law opened with a prelude explaining their triumph over the 'heavy yoke of the Romans). On the other hand, they were skilled agriculturalists, unlike many of the other itinerant tribes. Lastly, they were the first of all the Germanic tribes to convert to Latin Catholic Christianity, as opposed to Arian Christianity, allying them with both the Pope in Rome and the Byzantine Emperor in Constantinople (the Burgundians and Visigoths followed suit, which might have mattered if their kingdoms lasted).
So, of all the Germanic tribes, it was, ultimately, two of the more barbaric, the Franks and Saxons, the triumphed. The Vandals fell to the Byzantines, the Ostrogoths fell to the Byzantines and Lombards, who ultimately fell to the Franks much later, the Visigoths to the Arabs and Franks, and the Burgundians to the Franks.
The Frank and Saxons, for all their success, carried with them a fatal weakness, in terms of social stability: division of the territory (gavelkind or salic patrimony). The Merovingian and Carolingian Frankish rulers repeatedly divided their realm with every generation, as they considered their realm to be their personal property to be divided among their sons. And Anglo-Saxon Britain was nothing if not divided. A fragmented society such as this is not a stable environment
There were other reasons for such division, such as the inherent pragmatism of relying on the nearby lord for defense against whomever was coming down the road, and the tendency of officials to make their positions hereditary and landed.
Of course, the Frankish kings attempted to solidify their kingdom, and Charlemagne made many efforts in such matters, but, as we all know, his empire was soon divided as was traditional among the Franks.
Alternate History
So, lets say that Syagrius wins at the battle of Soissons and Clovis is killed. Most of the other Frankish rulers, as well as his sons, do not compare to Clovis in ability or foresight (one of the reasons that ultimately led to his dynasty's downfall), thus freeing Soissons from threats on that front and giving them some breathing room.
They might even be able to score some successes against the Franks or other barbarians in Gaul. The Franks will be fairly well divided and under sub-par leadership, making them an attractive target (as well as the greatest long term threat). The Burgundians also might be a good avenue for expansion: in addition to providing a contiguous link with Italy and the Mediterranean, they also are somewhat inclined towards Catholic Christianity (their last historical king was a canonized martyr).
Soissons thus occupies a position somewhat similar to Ostrogothic Italy: an effectively independent realm that nominally is ruling on behalf of the Emperor in Constantinople and seeking to maintain somewhat close ties. There are, of course, differences: Italy is richer and more urban, while Soissons is much more rural. On the other hand, Italy is ruled by Gothic Arians, while there is no cultural or religious difference between the rulers and ruled of Soissons. Geographically and politically, Soissons effectively takes the place of early Merovingian France, with a few key differences: the leadership is composed of more sophisticated Gallo-Romans, and they regard the ruler and the office as separate, avoiding the issue of the division of territory, avoiding the disintegration of the state.
Ostrogothic Italy is important for the development of Gallo-Roman Soissons for another reason: Theodoric's actions ultimately will have great influence on Soissons' fate. Historically, he attempted to unite as much of the Western Roman Empire as possible, arousing the ire of the Emperor in Constantinople, ultimately leading to the fall of Gothic Italy and all the problems associated with that (deterioration of urban Italy, the Lombard invasion, the economic costs in the Eastern Empire and the alienation of the heretics in Syria and Egypt). If Theodoric attempts such a policy, he may or may not seek close ties with Soissons. If he doesn't attempt such a policy, he'll likely seek close ties with Soissons, as another remnant of Roman civilization. If Theodoric tries to go it alone, then Soissons becomes the obvious foil for his ambitions, thus leading to an alliance with Constantinople.
Either way, we're basically replacing Frankish Gaul with a Gallo-Roman Gaul. We'll say that the Domain grows slower than the Frankish kingdom, but, due to its political institutions, is more stable and secure. Meanwhile, this stability will assist in the cultural growth of Gaul. When I speak of cultural growth, I'm pretty much referring to churches and monasteries, the main outposts of culture in early Dark Age Europe, particularly in the instances where the clergy and monks are literate. Its possible that a Gallo-Roman Gaul would have a wider cultural base, with the possibility of a literate aristocracy, but that isn't assured; we will simply assume that the aristocracy of this Gaul would be more literate than that of our history's Frankish Gaul was, and that the stability of the realm allows the clergy to focus on other matters, rather than constantly trying to civilize their rulers (the main task of the Latin Church in the Dark ages). Its this cultural base that is necessary to maintain the government.
So, as the Domain of Gallia, as we'll call it now, expands over the next few centuries, it forms a semi-civilized bulwark, incorporating the various tribes in Gaul as best as they can. The various members of the Frankish and Burgundian 'aristocracy' intermarry with that of the Gallo-Romans, as happened historically (though likely, with the Gallo-Romans ascendant, many of the marriages will be inverted; with Gallo-Roman sons marrying Germanic daughters), as the Church works to convert them to Latin Christianity.
The cultural strength of Gallia will prove a boon to Britain, as well. Without going into a long tangent, the cliffnotes version of the history of the Anglo-Saxon church is this: British clergy fled the Anglo-Saxons for Ireland, and converted the local. The Irish clergy came back and converted the Anglo-Saxons. The Anglo-Saxons then went and sent all their best minds and advisors to strengthen the Frankish monarchy (and convert the Frisians and Germans). The classic Dark Age standard of a Benedictine monk advising a king or lord often involved an Anglo-Saxon monk advising a Frankish lord. Then, the Vikings came and ruined everything.
With Gallia having a native source of educated clergy and aristocracy, the Anglo-Saxon clergy can focus their efforts locally, possibly even uniting England before the Vikings get there, so they can provide a unified defense (they might even get some assistance from Gallia). Or, possibly, they send their missionaries further afield, converting tribes beyond just the Germans.
Gallia will likely be closely allied with the Papacy, regardless of what happens with Ostrogothic Italy; unless the Ostrogoths manage to convert from Arianism, which would complicate things (but in a good way). This could lead to an analogue to Charlemagne's Coronation as Emperor. Except, now, instead of a semi-literate barbarian, it would likely be a literate Roman. Perhaps even with Byzantine approval (perhaps in exchange for military assistance against Persia/Arabs). Of course, with a Western Roman Emperor, Ostrogothic Italy, Visigothic Spain, and Anglo-Saxon Britain would technically fall under his domain, though they'd probably be as subservient to him as they were to Eastern Roman Emperor, historically.
Rough (best case) Timeline (just for fun)
Late 5th century:
Gallo-Roman forces prevail over the Franks, gutting the capable leadership of the tribe. A few of the smaller Frankish lords along the border submit to the Gallo-Romans. An alliance with the Burgundians to the south, who convert to Latin Catholicism from Arianism.
Early 6th century:
The Gallo-Romans push further into Frankish lands, taking everything north of the Meuse river. Several smaller tribes are re-settled further within Gallic territory, particularly on marginal land. The Gallo-Romans and Burgundians invade Visigothic Gaul, with most of Aquitania going to Gallia and Provence going to the Burgundians.
The Ostrogoths avoid becoming entangled in the war, to mollify the Roman Emperor in Constantinople. They instead ally with the Romans in their invasion of Vandal Africa, taking the rest of Sicily, as well as Corsica, Sardinia, and the Balaerics.
The Papacy remains firmly aligned with the Ostrogoths, who support the Pope, without converting. The Emperors in Constantinople maintain a general policy of religious tolerance, causing some friction with the Papacy, as they avoid suppressing the heretics in Syria and Egypt.
Late 6th century:
The last Frankish enclaves west of the Rhine fall to the Gallo-Romans, while the eastern tribes are conquered by the Continental Saxons. The Burgundian kingdom collapses into civil war and the Gallo-Romans take advantage of the power vacuum, incorporating most of their territory.
The Ostrogoths repel repeated incursions by the Lombards into Italy. To bolster their support among the population, the leadership converts to Catholicism, thus encouraging the Papacy to assist in the war. However, they are unable to hold onto Dalmatia, which falls to the Lombards. Meanwhile, the Saxons conquer the Frisians and Thuringians. The Visigoths, the only remaining Arian power, also converts to avoid diplomatic isolation.
The Eastern Roman Empire secures most of Armenia from the Sassanid Persians, and allying with the smaller states in the Caucasus.
Early 7th century:
The Gallo-Romans take the remainder of Burgundian and Visigothic territory in Gaul, solidifying their borders along the Pyrennes and Rhine. The conquest of the western Alemanni and the subjugation of Armorica (Brittany) completes the restoration of Roman Gallia. Gallia is the least urbanized of the three major post-Roman states in the west, but the only one that is governed by Romans. The various Germanic federates living with the territory are generally allowed to live under their own laws.
The Avars conquer a vast swath of land from the eastern Black Sea coast, along the Danube, and to the Saxon border. The Lombards fall under their rule intermittently, though the Avars have difficulty holding onto the region.
The Eastern Roman Empire is invaded by Sassanid Persia, seeking to regain Armenia. Due to poor leadership, the Romans face several setbacks and soon Persian armies are marching in Anatolia and reach Constantinople. Seeking assistance, the Romans ally with the Arabs, unified under their new religion of Islam. The alliance bears fruit quickly, as the Sassanids are crushed under combined forces, and are annexed by the Muslim Caliphate. However, relations quickly sour between the Arabs and the Roman Empire, and war breaks out between them, with parts of Syria and Judea quickly falling to the invaders.
Late 7th century:
The Eastern Roman Empire is hard pressed by the Arabs, who invade Egypt. At the same time, as frontier garrisons in North Africa are pulled to face the invasion, Berber tribes begin to pour over the border, and the Avars threaten the Danube border.
Meanwhile, Hispania, Gallia, and Italia are fairly peaceful and stable. The only major threat to Gallia is the Saxons, who are more focused on maintaining their border with the Avars (though they intermittently send raiding parties over the Rhine). The Anglo-Saxon kingdoms have congealed into only three kingdoms.
Seeking assistance from the cash-poor but soldier-rich west, the Eastern Roman Emperor sends envoys to the Pope in Rome, the Magister of Gallia, and the two Gothic kings for assistance. With the Pope acting as a mediator (in return for some theological concessions regarding the Pope's authority), the Emperor elevates the Magister to the rank of Emperor. The Visigoths and Ostrogoths officially fall under the new Western Emperor, though they maintain the same level of independence they previously held. Spurred on by the Western Emperor and Pope, the Gothic kingdoms join in the alliance.
Gallic forces are divided in two; half the army is sent with the Visigoths to secure North Africa, while the other half attacks the Avars in Lombard Dalmatia, with the Ostrogoths. The Lombards, unhappy with Avar dominion, ally with the Western Emperor and their kingdom is recognized as equal to the two Gothic kingdoms, contingent of their conversion to Catholicism. The Berbers, meanwhile, are offered a similar deal; though they do not hold as much power in North Africa, allying with them allows for the Western Emperor to simultaneously focus the military efforts on the Arabs as well as take North Africa from the Eastern Emperor.
Early 8th Century
The last remnants of the Arab invasion of the Eastern Roman Empire is removed, nominally restoring the entirety of the Roman Empire, minus Britain. The Arabs turn east, focusing their energies on India. The Western Empire, while theoretically containing almost as much territory as it ever did, is, in reality, confined to Gallia. The Western Emperors make no effort to expand their power beyond that, since they do not have the manpower or economic base to take either Italia or Hispania or even enforce anything beyond the nominal allegiance of either kingdom (or Lombardia or Africa).
The sole effective unifying force of the Western Empire is the Latin Catholic Church. Tied together by the clergy and Benedictine monks, the Church works to maintain the alliance between the kingdoms of the West, maintain their independence from the East, and, occasionally, strengthen the power of the Papacy (the clergy don't always feel more loyalty to Rome than to the local ruler).
Comparison to OTL: For the typical peasant, life is pretty much the same. The ruling class and clergy are more literate, which would ameliorate some of the worst parts of living as a dark age peasant; its better to live under someone who has some knowledge than just the best local warlord. However, life is still very much rural, compared to antiquity (with the exception of the Mediterranean coastal regions). On a political level, the component kingdoms of the west are, for the moment, more stable than their counterparts, and Italy is in much better shape. Western Germany is partially unified under the Saxons, who are taking the place of the eastern Franks of our history. Britain's doing about as well as in our history, and will have about a century to get its act together. The Eastern Empire is in very good shape, when compared to our history; Syria and Egypt have been damaged, but are still in Roman hands and are, comparatively, loyal as the Eastern Orthodox church and the Emperor have been more accommodating. The Western Church, however, will likely not approve of such accommodation.
The main remaining challenge to the west is the integration of the Germanic nobility (at least, outside of Gaul) and the combination of Germanic law and Roman law. This was not an insurmountable task for even our history, and the greatest Dark Age rulers made significant inroads into such matters; the rulers of this Dim Age should but up to the task. With any luck, they might be able to come up with a code of law that combines the best features of Roman and Germanic law.
Anyway, I'm out of steam. Enjoy the ride, pick things apart, argue and carouse.