@Zenon_pt: I beg to differ on a few points--I prefer to see Generals and actual political heads of state (or at least figureheads) as the rulers of countries. Having a country led by Confucius makes little sense to me, as well as a country run by Neils Bohr. Leave them as other Great People--that's why we have Great Scientists, Merchants, Prophets, Engineers, and Artists.
I think Poland is more of an outside shot, but I would through in King Casimir as another name to consider as a leader. His name probably already came up, but I haven't kept up with the 1000+ posts in this thread. (Sorry!

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I think Babylon/Sumeria, the Maya, and Austria are pretty much a guarantee. The Portuguese and Netherlands I would put in the second group, along with the Poles and the Byzantines--groups I want to see in, but not probably a shoo-in. That's a total of 7/8 (depending on whether or not Babylon and Sumeria will be separate) civs...more than they put in the last expansion pack.
Venezuela and Mexico I don't think have a reasonable chance. That's just me being honest, though--I doubt the Civ4 game designers will want to put them in over the civilizations mentioned above. And I'd rather have the Iroquois in before either of these civilizations--I think having been in a previous incarnation of Civilization should award that civilization some place in the game, even if it's in the second XP. Also, we have unlimited slots for civilizations in Civ4, so they can be added with little difficulty. As can every other civ mentioned here, but I think the top group have better shots than Venezuela and Mexico.
I would actually through out Brazil's name before Venezuela--Brazil is the current South American leader, if I'm not mistaken. They might make the final cut.