But the Colonies were administered completely differently than a the UK Dominions. Here the Emperor is absolute and governs in the traditional Roman fashion as the living embodiment of the Senate and People of Rome (SPQR). The Roman Empire initially treated native Romans from the city of Rome as full citizens and then gradually expanded citzenship to Latin Italians. Then its expanded into the Mediterranean treating the provinces as a means to sustain Italy which was seen as the economic and political core of the Empire. Over the centuries the contributions of the Roman provincials were recognized and the Empire expanded citizenship to be universal under Caracalla. The Empire treated all its provinces and the people residing within them as part of the Roman world. Now in the future, they'd be expanding this definition of Romaness to their colonies since Roman citizenship is not really based on ethnicity and race based caste system (like Spain in otl had) but based on a common cultural and religious identity. One could be Arab, Illyrian, Greek, Roman (Italian/Latin speaking) and still be Roman. The Empire is a multiethnic empire united around the common Roman ideals of one faith, one law, and one empire enshrined by Constantine the Great and Justinian the Great. Sure the colonies might be granted autonomy due to their distance from the mainland empire, but that doesn't mean they're de-facto separate countries. I think a more appropriate comparison from otl might be with Spain and its colonia viceroy system without the Racial caste system, or the modern US with its territories like Puerto-Rico having its population counted as Roman citizens.I think the best comparison is the Commonwealth, especially the UK and the White Dominions pre WWII.
Why would the Romans allow for there to be separate armies? They've had problems like this for centuries. Without tight control from the Imperial government, ambitous generals might take the opportunity to rebel. They've encountered this problem over the many centuries of their existence as a polity which makes it less likely that the Emperor would allow for separate armies. And with the populations of the colonies integrated around a common Roman idea, there would be no real reason for separatism since the local cultures and customs would be respected as long as they adhered to Roman law. This was radically different from the Spanish or British system. Spain had a racial caste system which alienated the colonial population while Britain had an increasingly democratized parliament where liberal ideas like nationalism and self-determinism spread. The Romans on the other hand are a multi-ethic empire based on the idea of a Universal Christian Empire that the Holy Roman Empire tried to puport itself as bur failed to in otl. The Romans would be more likely to assert control over its colonies while trying to integrate them as full fledged members of the empire like they did with regions they conquered in the many centuries of their existence.Long term I think the most likely answer is these areas becoming constituent Kingdoms within the Roman Empire. Part of an internal trade zone and restrictions on foreign policy but otherwise with full internal autonomy. Armies and Navies are separate but with the same equipment, training, and doctrine and possibly even a unified Joint Chiefs.
No real OTL comparisons but think along the lines of a really really tightly knit EU one step short of United States of Europe. There is still a France and Germany with a French and German armed forces but no one would bat an eye if a Frenchman was commanding a German division or vice versa and there is no difference between the makeup of a German and French division. In addition on the ground there is no internal border.
The Romans never really operated on that level of federalism. Even arrangements like the Exarchates of Ravenna and the Exarchate of Africa, was meant to be temporary. This was basically where the Emperor needed a more direct representative to handle a major military crisis. The exarch was a direct military field commander who was entrusted with power over the civil administration of the region. This allowed a more direct response to the crisis. Think of it like martial law being enacted and the military takes over the region. Italy was too far flung from the Empire and the Lombards were invading it. The Empire had other pressing issues near its core making it difficult to properly respond to crises in more far off regions like Italy. The Despotates to me at least, parallel the situation in regards to the old Exarchates. Plus the Romans were not operating a federal Empire. They were a heavily centralized and bureaucratic state with the army to back them up. They might impose client states in France or Germany initially since it would be easier than outright annexing them. It would allow the Romans to slowly integrate them as provinces later down the line. The Byzantines and the Classical Empire had done this many times in its history.No real OTL comparisons but think along the lines of a really really tightly knit EU one step short of United States of Europe. There is still a France and Germany with a French and German armed forces but no one would bat an eye if a Frenchman was commanding a German division or vice versa and there is no difference between the makeup of a German and French division.
@Basileus444 the circumstances that led to decolonization are completely different than the ones that the Romans are facing. The Romans are at their heart a mulitethic Empire with universal citizenship granted to those who abide by its laws. It unites various disparate peoples among the notion that there's one law, one faith, and one emperor. Though other versions of Chistianity are tolerated like the Copts, the Armenians, etc.Any equivalent to OTL decolonization is a long ways away, so I can’t say for certain since it depends on the details on the ground.
Southern Italy was basically organized into Katepanate since it was distant from the Empire proper which had a more eastern focus due to the hostile powers near Constantinople. Still the Catepanate was directly answerable to the Emperor with the central government still keeping a watchful eye over the Catepan and coordinating efforts with him.I think much of the decolonization ITTL would be people who now identify as Roman of an eastern variety, wanting their polity to switch from a Katepanate to a Despotate, so they have a more locally-responsive and autonomous government but still remain within the overarching Roman sphere.