Can't vote, but I would go for Africa. It wouldn't necessarily make for the best or most interesting gameplay, but I would sincerely find it refreshing. Sub-Saharan Africa has so far been severely underrepresented in any game worth of mention. (The only counter example I can think of would be Command & Conquer.) Not that I wouldn't love any of the other options. A few on which to comment:

Shogun 2: To sate my combined nostalgia and raging nihongofilia.
China (warring kindoms): Always found this period interesting. "Hero" anyone?

In addition to this I would like to see the 16th and 17th centuries covered. Maybe also something not thought of at all, for example the Stone, Bronze and Iron Ages (I'm an archaeologist ;)), Pacific Isles, Post-Apocalyptic Earth, whatsoever. Those would probably have to be modifications as they would come into conflict with CA's style. I also have a few ideas for a warcraft modification, again mostly due to nostalgia (war1 & 2).