Well, ok, let's get at it logically:
1) We have to look at new leaders, not civs, because
2) atm, we have 8 traits = 28 combinations, 2 unused. 1 new trait would open up the possibilities for 8 new combinations. A second new trait would open up 9 new possibilities. So, we'll have somewhere between 8 and 19 new leaders.
3) I doubt that they will double traits and that they give us 19 new combinations (which would essentially double the combos!). Also, it's quite difficult to find two new DIFFERENT and BALANCED traits (suggestions welcome!). That said, I wouldn't bet on any more than 12 new combos.
4)There are some civs who definately need a second leader: Rome, Egypt and Greece. There might also be thoughts of giving Japan, etc. a new leader and also to some new civs. Let's therefore assume that 5 civs get a second leader. 12 - 5 = 7 new civs (with unique units, etc. ... )
5)Most of you have given much more than 7 civ suggestion, so we need to see first the absolute essential:
Korea: Simply put, there are just too many Koreans playing computer games to ignore this civ!
Ottomans[/b]: A bit less wanted than Korea, but still a must.
Mesopotamia/Babylon/Assyria: Just look at the many threads that popped up in this forum
Other strong contenders are: Carthage/Phoenicia (first civ-mod for civIV!), the Hebrews (popular, judaism!), Khmer/Siam/... (South Asia), Scandinavia (Vikings if you wish) and all the others listed in that thread.
6)Got it. After the Vikings, we've already reached our limit! But here still are my personal favourites: Poland, Shapur II (for Persia), ...
mitsho