This is an idea that's been in my head for a few weeks now. I was wondering what might happen if the Crisis of the Third Century had truly been the end of the Roman Empire. I decided to create few maps and faction lists and see what might have happened. I was going to add these maps, but they exceed the site limits.
268 AD - Breakup of the Empire
POD - Battle of Naissus. The Goths are still defeated but Emperor Claudius dies. Aurelian still beats back the Goths and the Alemanni but is killed by the plague that killed Claudius in OTL.
The Roman Empire further breaks up into four successor states - the Central Roman Empire (Italy, Raetia, Noricum, Sicily, Sardinia, Corsica, North Africa), the Achean Empire (Pannonia, Dalmatia, Greece, Macedonia, Epirus, Thrace, Moesia, Crete, Cyrene), the Kingdom of Nicomedia (Asia, Rhodes, Bithynia et Pontus, Trebizond, Lycia et Pamphylia) and the Kingdom of Cyprus.
274 AD - Roman Civil War
The Roman successor states begin to fight amongst themselves. The Gallic Empire takes Raetia, Mauretania and the Balearic Islands from the CRE.
The Palmyrene Empire annexes Cyprus and starts pushing further into Asia Minor. Desperate, the Kingdom of Nicomedia annexes Colchis, earning them the ire of the Armenians.
Of the successors, the Achean Empire is hit the worst. It has lost Cyrene to the Palmyrenes, Pannonia to the Vandals and the Danube border is being flooded with nomads and Germanic tribes, in particular, the Goths, Gepids and Roxolani.
284 AD - Barbarian Invasions
In the West, the Romans are losing starting to lose their grip. The Alemanni return and seize Noricum and Raetia. Britain breaks away from the Gallic Empire and the North African provinces of both the Central Roman and Gallic empires break away to form the African Empire, seizing Sardinia, Corsica and Western Sicily. The CRE is now practically confined to Italy. The Franks arrive on the Gallic Empire's nothern borders and are settled in Germania Inferior.
In the Balkans the tribes broke through and tore the Achean Empire to pieces. While the Gepids remained in Dacia, the Vandals conquered Dalmatia and the Roxolani and Goths formed new kingdoms in Moesia and Greece, Macedonia, Epirus and Thrace respectively. Only Crete manages to escape, for now.
In the East, the Palmyrenes and Armenians finish off the remaining Nicomedians, with the Armenians taking back Colchis.
311 AD - Fall of Rome
The Gallic Empire was now reduced to Southern Gaul and the Alps. Roman nobles in Northern Gaul, disgusted at the Emperor's decision to grant the Franks land, drove the emperor out and formed a new state of their own - the Domain of Soissons. Spain followed suit and broke away, taking the Balearic Islands with it. The Franks took this opportunity to annex Belgica and Germania Superior.
The Alemanni pushed on into Northern Italy. In desperation, the Central Roman Emperor appealed to the Vandals and Goths for aid in exchange for land. Both refused, however, the Gothic commander of Epirus named Alaric (not the OTL Alaric) grabbed at this chance, not satisfied with raiding the coast of Asia Minor. In defiance of the King's orders, Alaric gathered an army and sailed over to Sicily, starting his work by driving the Africans out.
When Alaric's forces arrived at Rome, however, relations turned sour. The Emperor's refusal to hand over Sicily combined with Roman arrogance led to Alaric conquering and sacking Rome. After tossing the Imperial regalia into the Tiber, Alaric would go on to conquer the rest of Nothern Italy while his brother Theodoric sailed over from Epirus to conquer Southern Italy and the rest of Sicily.
These new conquests would forever divide the Gothic peoples. Those who had followed Alaric would be known as Visigoths while those who had stayed in the Balkans would become the Ostrogoths.
In the Balkans, the Gepids began to push into Moesia, leading to the Roxolani conquering Thrace. The Ostrogoths conquered Crete and Rhodes and began to raid coastal Asia Minor, destroying the Temple of Artemis at Ephesus.
In the East, the Egyptians, sick and tired of foreign rule, declared independence from Palmyra.
327 AD - Rise of Christianity
Ever since the crucifixion of Christ, Christianity had always 'been there.' With Roman influence waning, the Christians began to flourish and spread, especially in the Middle East, the Balkans and Southern Europe.
With the Palmyrene and Gallic Empires in decline, many states saw their chance to gain more land and power for themselves. In the East, the Roxolani crossed the Bosporus and conquering the northern coast of Asia Minor. The Palmyrene Empire lost Cappadocia, Sophene and Cilicia to the Armenians, Cyprus and Lycia et Pamphylia to the Egyptians, and Syria, Phoenicia and Palmyra itself to the Sassanid Persians who would also take Judea from the Egyptians. A loose collection of states in central and western Asia Minor claiming the Palmyrene mantle would linger on for a few decades before being conquered.
While the Britannic and Spanish Empires remained intact, the African Empire continued to lose territory. The Alemanni and Visigoths conquered Corsica and Sardinia respectively. Both Mauretanias broke away to form the Moorish Kingdom and the Berber Kingdom of Altava.
In Gaul the Domain of Soissions was conquered by the Franks and the Gallic Empire was conquered by a new tribe, the Burgundians.
The Alemanni and Vandals lost Raetia, Norcium and Pannonia to another new tribe, the Lombards. The Ostrogoths cease raiding Asia Minor due to competition from the Roxolani and to reconquer Epirus from the Visigoths.
370 AD - Storm from the East
43 years have passed and much has changed. The Sassanid Empire is now the undisputed master of the Middle East, having expanded its borders beyond the days of Cyrus, Darius and Xerxes. Armenia still remains but as a severely weakened vassal and buffer against a powerful new enemy from the north.
In Europe, the arrival of the Huns threw everything off-balance. Several tribes were pushed further westwards or, like the Gepids, annexed completely. The Lombards took refuge in Venetia, driving out the Alemanni. The Angles and Saxons took to the seas and started to carve a new homeland out for themselves from the Britannic Empire. The Suebi travelled through Frankish territory and joined up with the Burgundians who had been driven out by the Franks. The two tribes moved towards Iberia and became settled in the North and South, confining Romano-Spanish rule to Lusitiania.
While the Ostrogoths and Roxolani entered into a period of consolidation, the Visigoths continued to conquer, taking Corsica and Segesta from the Alemanni and the remnants of the African Empire, except for Numidia and the Berber Kingdom which were conquered by the Moors.
395 AD
The Persians are consolidating their gains, realising they may have overstretched themselves. The greatest source of disturbance comes from alternating periods of toleration and persecution of Christians, the latest act being the final annexation of Armenia.
Most of the activity going on is in Europe. Lustiania is now the only place left where people call themselves 'Romans' even as the Burgundians push at their borders. Britain is now firmly under Anglo-Saxon dominion despite the kingdoms and tribes not being united.
The Franks have conquered the remaining Alemanni. Fearful, the Lombards pushed deeper into Italy, taking Corsica, Liguria, Etruria, Campania and Apulia from the Visigoths.
476 AD
In OTL, the Western Roman Empire had 'fallen.' In this timeline, it is both the Western Roman Empire and the Sassanid Empire which have now 'fallen.' The Sassanid dynasty is now confined to Babylonia and Charax Spasinou. The great empire they once ruled has broken apart due to religious tensions, poor leadership and overextension. From the chaos emerged seven different successor states - the Kwarizmian Empire, the Arabian Empire, the Kingdom of Georgia, the Kingdom of Pontus, the Kingdom of Armenia, the Jewish Kingdom, and the most successful, the Neo Egyptian Empire, which has swept across North Africa, taking Carthage from the Visigoths and annexing the Moorish Kingdom.
In Europe, the Burgundians have conquered Cantabria from the Suebi, the Balearic Islands and Lusitania, ending the last remnants of Imperial Roman rule. The power of the Huns begins to wane and they start to get pushed out of Europe, losing Pannonia to the Vandals and most of Germany to the Franks. Inspired by this success, the Franks push into Italy, conquering Rome itself. Southern Italy remains in Lombard hands while Sicily is all that's left of the Visigoth Kingdom.
I know this isn't the best alternate history scenario ever wirtten, but it wasn't really meant to be. If there are questions, any glaring errors or anything else I'm missing, please let me know.