I doubt that any of the 18 will be civs that we haven't seen in previous official releases. Ideally, I'd love for them to be spread across the continents fairly evenly, but I expect that they will be mainly European as normal.
EUROPE
1. England
2. Germany
3. Rome
4. Greece
5. Russia
6. France
7. Spain
MIDDLE EAST
8. Egypt
9. Arabia
10. Persia
AMERICA
11. Inca
12. Aztec
13. Iroquois or Sioux (I hope)
ASIA
14. China
15. India
16. Japan
17. Mongolia
RANDOM
18. United States of America
Personally, I'd prefer to dump the USA in favor of Ethiopia, change Arabia to the Phoenicians, and even merge Greece with Rome to make room for the Mali, but I very much doubt that Firaxis would go for any of that.
EDIT: I'd also ideally like them to have Leaderhead personalities and graphics that change automatically with the Era. For instance, Rome could start out with Scipio, change to Augustus, then Justinian, Medici, Garibaldi, and Mussolini. Persia could start with Cyrus, change to Shapur II, then Al-Rashid, Abbas I, Qajar, and Khomeini. The bonuses / traits wouldn't even need to change, just the feel of the regime's policies when playing against them. This too, is probably too much wishful thinking though.