IMO There are only four qualifications for civilizationdom.
1: having a complex civilization (Maya)
2: Being a factor in a key event Huns and (hopefully Zulu)
3: Being an important empire
4: Being romanticized
With that I say The Khmer, Ethiopia, and Sumer
Another ancient civ like the Assyrians, Akkadians, or Hittites for instance, would be much more interesting to me than Sumer. The Akkadians would not be much different either, but they have not been used before. Its just that playing Sumer in civ 3 and 4 to me was boring. Anyway, we already have Babylon we don't really need Sumer. I hope they suprise us with something new and interesting.
Not sure why, but I have a feeling that civs that have been featured as cameos in scenarios probably won't be in the main game (possibly because it would conflict with their abilities/units in those scenarios). That rules out the Normans, Norwegians and Saxons (1066), Hittites and Sumeria (Wonders of the Ancient World), Hiva, Tonga and Samoa (Paradise Found), or the Jurchens (Samurai Invasion of Korea).
Having said that, checking the Wiki Byzantium was in the Mongol scenario (which makes sense), so I could well be wrong. And admittedly, of the above only the Hittites, Sumerians and possibly the Normans are plausible civs to add to the game.
I believe the Mayans get Calendar related bonuses, not sure what it exaclty does.
Do they have to end the game by 2012 rather than 2050, I wonder?
(Yes, I know the Mayan calendar really does include dates post-2012, but that would spoil the joke).
Im thinking they could make more Scenario focused expansion that would also add more stuff to the future era. There are lots of possibilities for scenarios. They could also add more leaders to existing civs. Random events could also be added allthough I doubt it.
I'm disappointed there aren't more scenarios included. I'd eventually like to see scenarios representing each of the civs in the game, though that's unlikely.
What I would like to have is mercenaries and/or unique units for militaristic city states.
Unique units would be a good idea and tie in well thematically with the unique luxuries from mercantile states. I doubt it will happen in the expansion, though - the new unit roster seems pretty likely to be filled with new general units and at least 9 UUs, without space left for a UU for each of (presumably, although Tyre is turning mercantile and Edinburgh, and possibly Dublin, will have to be replaced by a new state) half a dozen militaristic CSes.
do we know the mayan ability? if not, they could fit with jungles. and to be fair, jaguars get a jungle bonus, even though they're a uu and not a ua.
The trouble is, jungles are poor tiles to work until Education - it would penalise a civ to give it a bonus that's reliant on settling near lots of jungle. And giving them a major resource boost for using jungle would go too far in the other direction (particularly since getting a science bonus from working them later in the game is a boost that needs to be compensated for by low production in other respects).