How about "none of the above?"
1) Vampires are VERY overdone.
2) Greek mythology might be a bit awkward when you already have not only a European based civ, but another Medittearean civ (Vinci, modelled on Italian city-states). Besides, Age of Mythology already did them.
3) Samurai VERY overdone as well.
4) China has some promise, but think one Asian culture might be more interesting.
No offense to anyone, but here is an idea that has been mentioned before. Anyone ever heard of the "Kahan?" They were going to be the forth race, but were removed.
The culture would have been based on the Mongolians. Mongolians are VERY bad-ass historically. Even moreso than all the historically inaccurate samurai, vampires, and the like combined. Folks like Genghis Khan are just awesome, after all. (The title "Khan" is the obvious source of the civ's name.)
The Kahan likely would have built cities on the backs of giant creatures. However, this was hard to balance. Some influences of theres did make it into the game: the Berserker infantry, the Berserker Rider cavalry, and the Spiritchief (IE Yontash's class). If you open the editor and look at the "Empty civ" you can see a Kahan city guard would have looked. (Guy clad in furs).
Originally, Yontash was going to be one of their heroes. Even Yontash's in-game description mentions "Far beyond the Cuotl jungles" or something along those lines.
The ideas for a master unit would be cool: Perhaps a city-carrying beast could be "unleashed" and rampage around. The master unit would likely be on land, as well.
Ice magic, I think, would have played a large part in their military. There were going to be originally from the frozen "Kahan Wastes" (mountainous area above and between Cuotl and Alin lands).
Another idea:
A Polynesian based civilization with Lovecraft influences. (Google "Call of Cthulhu" or "Howard Philips Lovecraft" if you don't know what I'm talking about.)
They would likely have mind control and water related powers. (Might serve as a problem, as there is not supposed to be any naval combat and programming navies into an expansion or mod would be a pain.)
However, perhaps units might be giant sea creatures and boats encased in magic spheres/bubbles of water so they can "move" around on land, and air units resemble flying Easter Island style stone Moai heads.
Ideas for a name would be "Mu" or "Lemuria" after fictional lost continents, or even "R'lyeh" for a direct Lovecraft reference.
These are my personal preferences for if they (or any modders) put in a 4th civ (and balance it well): 1) Kahan, 2) Mu/Lemuria/R'lyeh, and 3) Chinese or Korean based nation.
Indian might also be interesting, but they might be "too close" to the Alin in terms of "architecture" and might end up looking like Alin minarets+smoke stacks (might be interesting, might not).
A Korean based nation might have a special power relating to entrenchment and strong fortification, perhaps tunneling.