What is this obcession with making Civ more realistic? Civ has never been about realism. I mean, c'mon! Romans with nukes? Aztecs with nukes? Puh-lease!
Civ has been about historical-based gaming, as Oda N. said. it's not about accurately re-creating history. Every game, following our history? That would be boring for me. Truly, it would.
Civ started with an idea, a theme, if you will. That theme was expanded on in Civ2, taking the franchise to new heights. It was further expanded on in Civ3, doing things with Civ2 that wasn't thought of, but probably wished for. In Civ4, I'd like to see a leap into new aresas that we've thought about, but don't have. Realism? It's as real as it needs to be. Historical? Again, it's as historical as it needs to be. I don't want to get trapped into a certain mindset and have to play a certain way simply because historically, that's how it happened. Imagine if the French had fought with the British against the Americans in the Revolutionary War. Imagine if Japan came in on the Allied side, and Pearl Harbor never happened. With Civ, it's possible to do that, and to explore possibilities.
I've never played on an Earth map, nor do I intend to. I like the randomness that can be created. I like not knowing where things are going to be, and who my opponets are. I like fighting against one civ for most of my 6000 year reign, then need them to help kill off another opponet and get that win.
I say let Civ4 be as real as Civ3 is. Give us new concepts to play with. I'd much rather see that, then to have the history of the world painstakingly redone.
After all, it's a game. It doesn't need to be historically accurate.