My 2 cents, just for the heck of it:
My heart says to include Kongo. It's just too cool not to!
My head says to include the Majapahit or Sri Vajaya (Indonesia is a 230-million-strong nation today, so who wouldn't want to sell games there? Also, these empires are neat!)
I would accept the inclusion of Brazil - South America is under-represented, and Brazil isn't too much younger than the USA. Also a huge potential market.
I would not be happy with another European civilization, especially in the West or even in the Far North or East. I just think we have enough!
Tibet would be amazing, but controversial in the world's largest gaming market - China. Israel would be a blessing, but controversial in the billion-person market that is the Mid-East. Another Native American Civ would rock, but I don't even know who that would be, since there are scores of equally-deserving candidates. Zulu would be OK, but if I ever have to start next to Shaka in a game of Civ ever again, I may start crying.
Whatever happens, I am excited for today's announcement
Disclaimer: These are just my hunches/opinions and I make no claims about the moral rightness of including any of these.