I'm rather happy on the way Firaxis has spread the civs on BNW, despite having only one sub-saharan african, and the usual eurocentrism, they managed to cover most of the world (even latin america), of course assuming the 2 last civs are Venice and Shawnee, or any native american.
So here's my list of civs I'd like to see in a third expansion.
1-Kongo
2-Vietnam
3-Mexico
4-Mughals
5-Israel
6-Sicily
7-Comancheria
8-Switzerland
Kongo is a must, everyone was surprised when they werent in BNW, hopefully by then we finally get sub saharan city graphics. Female leader with Queen Nzinga, thats always a plus.
Vietnam, also a good contender to be in BNW, interesting culture, recognizable by US audiences, could take advantage of the ideology system. Another female leader with Trung Trac.
Mexico, Brazil opened the door. Mexico has a rich culture, is very marketable in both latin america and the US, and even if I think Bolivar would be cool, when Mexico was breaking apart into smaller nations, it actually managed to put itself back together. (unlike Gran Colombia), if colonization and revolutions are part of a third expansion Mexico fills both those criteria.
Mughals, I'd love for India and Mughals to be diferent civs, India could as well be reworked like France to allow for them. Military and cultural, maybe with a bonus to reducing the penalties effects of diferent ideologies and religion in the empire.
Kingdom of Israel that is, we could certainly use another religous civ, centered more in the idea of holy city and strong cultural resistance, and less on spaming your religion everywhere.
Sicily, because Europe has to be in the game, a naval crusader civ, bonus against diferent religions, able to call on holy wars and recieve military support from civs or CS of the same religion. Would be really interesting, your best friend or worst nightmare.
Comancheria, (unless the Pueblo make it, but I doubt it), steppe raider civ, largest territory in NA. It would be interesting if they had bonus to pillaging resources and caravans. If Firaxis devised a way to depict their nomadic lifestyle it could be really interesting.
Switzerland, because Europe. A diplomatic, trade civ, bonus to great improvements, despite the new diplomatic game, there's no civ that really focuses on taking advantage of the world congress. It would be interesting how they would deal with ideologies, maybe you could really stay neutral, and play each block against the other while pumping out GP.