I think that the main problem is that most civilizations don't evolve in such a natural manner from one nation to another. I could see the Han becoming China, Persia becoming Iran, the Celts becoming Ireland or Scotland, the Greeks becoming the Byzantines and maybe, maybe Rome becoming Italy. Beyond that, they make little or no sense except in terms of geographic location.The Iroquois, for example didn't give rise to Canada, and where would the United States come from? Moreover, where would the Sumerians and the Hittites go? Also, some civs kind of 'branch-off' into many different civs.