@Skallagrim
1. The Rhineland is just as much an integral part of the Roman empire of the classic era as was Africa. In the minds of a northern people within the Latin world, certainly even more so.
2. I do not believe Italy will cause issues in and of itself. The issue of otl, was never Italy, but a battle of authority between the Emperor and the Papacy over control over the Translatio Imperii. Italian states simply took sides in this civil war in the Empire. A civil war that the Papacy decisively won. As long as these Frankish monarchs respect the Papacy, which they will be doing if they truly seek to expand exterior lands, instead of engaging in Roman bloodletting with the Papacy, then the Papacy will not rain down war upon them. A good way to do this too, is avoid any intermingling between the Byzantine and Frankish court and also creating earlier precedence toward expansionism against the varied enemies.
3. Germany was kept in existence due to its prominence over France in otl in terms of overall power. It was the Empire that was restrained in otl by the Papacy and the true protector of the France in otl, was the Papacy, with whom we find the true victor of most of the defeats that the Empire suffered. Without the Papacy managing matters, the Empire may well of swallowed the West Francian state in time. Papal reasoning for this is clear. The Empire in the 10th and early 11th century attempted to overtly dominate the Papacy and revoke the Donation of Pepin by force. Such matters were not accepted and the Papacy rapidly changed its tune, from a pro-East Francian stance, to a pro-French state, whom it used as its bargaining chip and its battering ram to threaten the Empire. Presumably too, the Papacy envisioned the Normans as its benefactors against potential Imperial aggression, hence the Papacy granting them titles and sending them to and fro in conquest sprees. This of course would backfire, yet it still displays that in the Middle Ages, we are seeing a different scenario, wherein the Empire of otl was in very early years, at a soft war with the Papacy.
In this atl, West Francia, the more powerful of all the divisions of the Frankish realms, will have at least a short window of Papal support. With that, they will take initiative and assume direct lordship over the East and reunite the empire. This to me, will be assured, unless, as I said, the Papacy decides to truly throw itself in front of the Western momentum and thus save the Eastern Frankish realms.
4. I also dispute the notion that in this period, that Rome had anything to do with a geographic realm. Papal opinion of the period, in my understanding and interpretation of the texts, seem to imply a universal world spanning empire conquering and subduing all peoples. This was viewed as under the canopy of Papal monarchy, but certainly, the Papacy held firmly that there was but one Lordship on the earth, that of the fountainhead of Europe, the Frankish monarchy. Which was intermingled and undistinguished from that of the Roman Empire.
Indeed, as I mention in other threads, all empires (which Latin/Germanic Europe composed itself of in the Middle Ages) have an overarching mission. For this matter, Western Europe in this period, composed of the Franks, Lombards, English, Papacy and Venice, molded the concepts of the Roman Empire, with Christianity and the Frankish noble customs and prestige to create an Imperial Mission. A mission which saw as its goal ultimately:
-The creation of a holy realm devoted to the Triune God. This is a clear cut topic. Promote the correct cult practices throughout the civilization. Amounting the necessity of the Papacy and a massive church structure with which to spread correct practices and also to instruct others in correct religious discourse. Likewise, the consistent practice of Church Councils amended to this needed activity in the civilization. It would eventually come to a head however as these disputes became, what is truly correct cultic practices? This is the Reformation.
-To export Frankish/Germanic/Latin nobility unto new realms and hence expand feudalism indefinitely across the planet. By this, we refer to the conception that European expansionism, was predicated upon an exportation of feudalism and Frankish nobles unto all lands who submitted or the cognate Papal mandated people group. In all areas conquered by the Latin realms, without exception, feudalism of the Frankish model was implemented to some degree and every lord maintained a distinct Frankish or similar posture. The best examples of this, is Sicily, Iberia, Outremer, Prussia, Mecklenburg, Brandenburg, Bohemia, Ireland (to a degree), etc... This is critical in that seemingly, the Papacy envisioned a world ruled under its grip of ever-expanding influence, south, west, north and east, using the expanding presence of a virulent and warlike Frankish/Norman nobility for which to assert correct cultic practices.
-A world conquest agenda. While it is likely that case that this was less virulent and forceful as that of the Umayyo-Abbasid conception of world conquest, it is nevertheless important to mention. Mario Liverani describes European expansionism in the Middle Ages as a sort of world conquering revanchsim. One of restoring Papal and Frankish/Roman hegemony over the 'planet,' which had been lost to them due to the treachery of the Greeks and the aggression of the Arabo-Islamic world.
It too, was not only a defensive and or reactionary act. Yet, one of true universal claim to the universe around them. Court poets of Charles I and other documents from that period display that the Emperor felt himself to have been 'a great star' the sole king. Likewise, the Papacy was framing itself at the same time as the canopy upon which it could direct a conquest of the world. The Papacy is the greatest example of this, as every Pontiff from 1040 onward would iterate, the Papacy is lord over all and that the only true role of the Papacy and the states surrounding him, is the expansion of Christendom forward. As the famous, yet later carol, Riu riu ciu mentiones:
'Hear the song of Peace He brings to all the nations, calling all to join. Join their hearts in jubilation. No more needless wars, no more hunger for the poor... Let the World Unite, helping one another more!'
In fact, a constant in Medieval popular and religious song, is the notion of uniting the world religiously, which means implicitly a political unification as per Papal edict.
'A
splendid star shining upon the mountain as
sunshine. Hark (!), the multitude assembling,
ALL people rejoicing, the rich and the poor, the greatest and the pettiest are gathering as well upon
our mountain, witnessing
together. Let us herald once more this: Hail Mary!' - Stella Splendens
The splendid star being the message of peace, the correct and true religion/cultic practice. Hence the mountain becomes the Church and the multitude being those who have been subsumed into the wider expanding Latin/Frankish realm.
With the tangent out of the way, my opinion, is that the idea of a Roman Empire, was conceived less in terms of a geographic border, but of a universal empire that embodied at least to some degree the above. When Dante Alighieri made his argument in 'De Monarchia' he makes odd references to the King of Germany or the Emperor of Rome, not as simply the Lord of Italy, but the implied Divinely appointed master of all other kings across the world. His role was in the exportation of this empire and all of its trappings to the entirety of creation, as Dante felt Rome had done. In this sense, Rome was morphed into a universalist Christian Frankish empire that was envisioned to ideally encompass all things.
And surely, I doubt that the conception of the Roman empire had as much to do in the year 1000 CE to Trajan, as it did to Charles I.
5. I do not believe that it was legal according to Medieval custom to exchange vassals in the manner you mention. There is no precedence that I know of for a 'vassal trade.' Wherein one realm exchanges vassals for other vassals in a sort of card trade. Vassals were formed through a series of oaths and years of customs that bounded lieges to vassals. They were not so easily ruptured, for the vassal made his vows directly to monarch, thus the vassal was an active participant who could reject transfer if they pleased.
It is also a breach of oath to exchange a vassal for another. It would break any pretense of land ownership in the realm. Why would lords pay their service to the lord if he is permitted to exchange them about as a pack of bureaucrats or eunuchs? Especially such a prestigious Frankish realm as Flanders, will not accept such a transfer.
Now, there is precedence in Medieval custom for ceding vassals and titles in wars, Likewise, there is precedence for the Papacy having the legal authority to transfer vassals to and fro or excising vassals from a realm (this is what occurred to Flanders during the Pontificate of Boniface VIII).
Flanders is also the last realm to be ceded. It is one of the heartlands of the Franks, there is little chance that it is ceded, even assuming that it is legal according to the customs. Ultimately, the Frankish lords of West Francia, will refuse to cede any of Middle Francia to the Eastern Franks. The real question though, is not if the atl Empire cedes lands to the EF, the question is rather how will the EF deter this enemy. They will have to appeal to the Papacy, which I can imagine in this atl, giving the freedom to take the realm as theirs.