If there is only one "Egypt" and only one "China" in the game, each of which lasted about 3 millennia, and an "India" that lasted two milennia, a case can be made for only one "Rome" that lasted two millennia. While I acknowledge that examples like Scotland and Nubia leave open the possibility that we might still get two "Roman" civs purely for sake of marketability, I think it is somewhat eurocentric to be insisting that Byzantium be a separate civ while showing extremely little interest in how the Mughals or Chola or Old Kingdom or Manchu are portrayed in the game. We have absolutely no mention of Burma or Vietnam in VI, each wholly distinct from Khmer--more than the Byzantines were from Rome--and each lasted nearly as long, and yet I see very few people complaining about how poorly SE Asia is represented. But there's always some myopic "where muh Bizants?!?" thread of conversation being started somewhere.
It's not that people want Byzantium that irritates my camp, but the sheer entitlement that accompanies these sorts of threads. The fact is that the design of VI so far could accommodate either way. We could get a Byzantine civ, we could get a Roman alternate leader. Some of us would prefer the latter because we want to see design space opened up for fresh blood: Italy, Bulgaria, Romania. And although I am advocating for Bulgaria and think it's a perfect fit for VI, I'm not demanding it like Byzantium fans do. We don't actually need Byzantium to be its own civ, nor are we guaranteed it after Carthage got blobbed into Phoenicia and England has a leader who speaks Occitan.
Let me grant you some assurances though. Byzantium is stupid popular; the obsession of every sophomore history nerd. If by some chance Byz fans don't get what they want, there will be a mod. And the support for it will be so strong that it will likely be the best mod in the game, designed from the ground up, full assets and animations, extra uniques, maybe even better than the official Firaxis civs. Maybe even with custom recorded music. Byzantium will probably be fine.