I have said I a couple of times and I say it again:
Given the fact that the USA are in the game this discussion is utterly pointless.
Edit, OK,Europe is overcrowded, but I wager most people play on random maps anyway....
The USA is a mix of multiple european civilizations, with aspects of african societies added into the mix via the slave trade, and indian societies via many mixed marriages and the continuing presence of reservations as independent units within the society setting it far apart from it's "supposed" english roots.
American civilization also, in my opinion, encompasses canadian and mexican states. Not the USA, though most leaders are drawn from that country due to it's dominance in the area over the past five hundred years.
You can't really seperate the US out from "american civilization" as the areas history is so tightly tied together, the split with canada is very recent and not really "complete" a lot of people live in canada and work in detroit, and california, texas, and new mexico have almost as many actual mexican citizens as american ones (A current political debate) and a LOT of mexican ancestry, most of that area was mexican within the recent past.
North Amereican Free Trade is also bringing the area closer together lately.
You probably could call "American" civilization European civilization, since it has so many influences from different areas. Mostly english, french, german, irish, and Italian. But that would be rude the the african americans, the native americans, the increasing "hispanic" element, and the chinese immigrants who did so much work on projects such as the trans continental railroad.
It's a whole different argument, and one that is a LOT easier to understand than the concept of including "Byzantium" as a seperate civilization with it's greek and roman roots.
It could easilly be seen as a continuation of either culture, with rome being dominant gameplay-wise as in game terms the "greeks" were conquered and obsorbed, contributing their own part to the larger "roman" civilization . . . And certainly NOT continuing to exist and evolve seperately the way america's colonial parent nations still do in "europe" today.
It's also easier to understand Including America than some countries that are only SLIGHTLY older, and cover a much smaller area. For example the SPANISH netherlands (either half) or portugual (not including brazil).
Not that I want those civilizations out of the game. I'm just pointing out that the north american civilization makes more sense to include than those two mostly spanish states.
I would argue FOR the inclusion of Brazil which covers most of south america and has it's own larger non-european cultural elements due to it's mayan and other indiginous influences. Except that if brazil was included portugual would probably need to be taken out and merged into general spanish/iberian civilization.