They're obviously going to include 1 or 2 more European civs, because of the whole "Scramble for Africa" scenario and the focus on culture.
1. Poland
2. Assyria
As for the rest:
3. Portugal - This seems a likely choice, because they were an influential European and colonial power, with a presence in Africa (Angola, Mozambique) and South America. Lisbon is already a city state, but that wouldn't deter them from putting it in.
4. Zulu - Another likely choice, since Shaka is a Civ staple and a favorite amongst fans. Plus, the game is lacking in African civilizations, and this would be the most popular option, since we have nothing in the southern part of the continent.
5. Belgium - Another likely contender, especially because of their not-so-positive legacy in the Congo and their controversial involvement in the scramble for Africa. It's the only other major European power (except for Italy) that has yet to be included.
6. Brazil - Not too sure on this one, because it's a fairly young country. I know America was included, but America has had (no offense to the Brazilians) a pretty big impact on the world in its short existence. But it is the foremost power in Latin America, so I could see it being a possible option.
7. Venice - Not really an empire, but definitely more influential than the likes of say, Polynesia, so that's an option. They could include places like Dubrovnik and Ragusa as secondary cities. I put this one as a more probably option than Italy, because this expansion seems to be focusing a lot on culture and trade routes, both of which the Venetian Republic was very famous for.
8. Indonesia/Majapahit - More Asian powers are needed, and Indonesia is a big one that has never been included. Again, a very new country, but if they included the Siamese empire, why not?
9. Sumeria - A very old empire and possible contender, since they were included in Civ 4. The Middle East has a lot of empires that were not explored.
Other possible options:
The Maghreb or the Moors - Maybe too similar to the Arab empire, but still, a possible option for Africa, although Cordoba and Granada are already taken by the Spanish empire. Plus, Isabella's screen is pretty Moorish in appearance already and may be too confusing
Italy - I don't know how they'd get this to work with Rome, since the city names are very similar. Rome is the most influential city in Italy and in the Roman Empire, so they'd maybe have to call it "Roma" or something. But then they'd have to do things like Milano, Firenze and Venezia as well.
Congo - Not too informed on their history and whether or not they'd be too similar to the Zulu, but they're another option as an African civ. Having both them and Belgium included could be interesting.
Mexico - Again, a new country and Spain, the Aztecs and the Mayans, all of whom made Mexico, are included. Maybe it would be too much. But if Brazil is an option, so is Mexico.
The Mughal Empire - India is already included, but that one seems to be more culturally similar to the British Raj than the Mughal Empire. Again, the overlapping of city names would be an issue.
Khmer Empire - Maybe too similar to the Siamese empire?
Holy Roman Empire - I don't think this one was too popular with people in Civ 4, because it was too similar to Germany.