I mean it's always possible, but maybe not probable.
Either way I think the Byzantines will be their own Civ but there is always the possibility that a leader, like Justinian, could possibly lead Rome in addition, since he successfully reconquered Rome and the lands around it and codified the Roman laws. If the leader ability is good and fit's into Rome's playstyle I'm for it.
Though I agree it would be harder to implement for the later rulers.
It is the later Byzantine Empire that is most interesting because it is less like ancient Rome and its achivements are more impressive due to more limited resources and much more capable enemies.
Just had a minor realization. Eleanor leads an industrial-heavy England. England doesn't feel "Angevin" at all, but for whatever Eleanor brings to the table. Yet the devs were fine with a very culturally different period of England being blobbed into England proper.
I know people really care about getting the flavor of Byzantium right, but...I think there is a reasonable case to be made that the devs would be just as fine with a Byzantine leader for Rome as they were putting Eleanor in the game.
I've already said I wouldn't miss Byzantium as a separate civ and would prefer Bulgaria. Some small part of me would be really pleased to see no Byzantium in NF since it seems like it won't be the alt leader in expack 5.
The happiest medium for me, personally, would be to make a Byzantine leader for Rome, but beefier. Give the leader two uniques to really drive home some Byzantine flavor. Then maybe give Trajan a rework by giving him a new unique (which he kind of has already with the monument). I really don't see the point of wasting a roster slot on Byzantium when the Roman uniques see a very good foundation for them, and if any civ deserves to have leaders have a bit of feature creep I think Rome deserves it.
They created a whole civ for just Alexander the Great and the gap between the Romans and Byzantines is probably much more significant than between Greece and Macedonia. The Byzantine Empire was maybe the most important medieval european state in terms of political and military influence.
I always see Alexander and Cyrus as effectively being dual leaders for two very culturally/geographically related civs. I don't really think of him as very Greek so much as representing the western influence on the general "Persian" region. Which is why I would also think it would be rad to get the Timurids to complete the trifecta of cults of personality.
Byzantine Empire was maybe the first state in the world to develop sophisticated military theory and probably had a much more sophisticated take on warfare compared to the Roman Empre to make up for its limited resources and the fact it often fought more powerful enemies while the Roman Empire at its height was only threatened by civil wars.
Okay, military success and innovation is fine and dandy, but
most empires were militaristic and we already have many domination civs in the game. I find this the most boring argument for including a civ, because it either completely forgets that the devs have to make the game fun, or elsewise indicates an extremely narrow idea of what fun can be.