Regarding proposed playable Byzantine civ spawn.
As I currently see it, if the AI Greeks survive, then players will just see a weak/normal Greece. If they don't survive, players will see a strong newly spawned Byzantines civ that behaves completely different from the Greek civ with new UU, UBs, leaders etc. I feel this makes the game unpredictable in a not very good way.
I can't remember who posted the awesome pics of a historical Roman Empire in RFC, but I recall afterwards you said that you'd look into having Rome represent the actual Roman Empire as opposed to just the Western Roman Empire. Are you still planning on doing that?
In summary... ways of including the Byzantines in 3000BC in my order of my preference
1) Between Rome and Greece, whichever civ completes the a GrecoRoman civ specific quest (such as Control Byzantium and at least 2 cities in Europe and 2 cities in Asia in the year 600AD), gets
-their palace automatically moved to Byzantium
-couple of free military units spawned at Byzantium
-access to Byzantine unique units, unique buildings,
-and the ability to have the most challenging Byzantine UHV condition count towards meeting one of their 3 UHVs.
2) A combined Roman-Byzantine civilization. Buff this civilization. Automove their capital to Byzantium when certain conditions are met. Give this civ 4 UHV conditions (2 from Roman Era and 2 from Byzantine Era). Will need to attain 3 of 4 UHV conditions for a UHV.
3) A combined Greco-Byzantine civ. Same as above but with Greeks instead of Romans.
4) Kinda like the proposed "personal union between France and Austria/Germany/HRE to represent the Carolingian Empire" idea. Except have a personal union between Rome and Greece
5) AI controlled Greek civ is entirely replaced by Byzantine civ in every game at Byzantine spawn. If Greek civ is human-controlled, then player gets option whether to spawn and switch to the Byzantine civ or not.
8) No Byzantine spawn in 3000BC (like in RFC).
6) Byzantine spawn only if Greek civ is already defeated.
7) Byzantine spawn no matter what.