Ok so we know 4 already (Maya, Inca, Hittites and Sumerians) and we can only guess for the 4 left. For a civilization we really need a good animated leaderhead and a UU, but those leaderheads that fit in the game with all the others are hard to find, the rest (cities, heroes etc.) can be put in by the editor.
My personel choice would be:
5) the Dutch, I'm dutch
, and in the 17th century the Dutch civilization was a world power and had colonies on all continents and is still today an economic power even if the Netherlands itself is a small country.
6 the Portuguese, In the 16th century a world power and also colonies all over the world and Brazil once a portuguese colony is the 6th most populous country in the world (in south america there is more portuguese spoken than spanish!)
7 the Polynesians They were a power in the pacific and spread from the east of what is now indonesia all over the pacific ocean, to Hawaii and to Tahiti and outendly to New Zealand (a dutch name by the way) were they gave quite a fight to the english.
8 the Aboriginals ok when they met the westerners for the first time they were still in the stone age and were all different tribes really. But they spread all over the fifth continent and lived there isolated until the 15 hundreds or so. (who knows how they would have developed if they weren't so isolated from the rest of the world, this is a what if game after all!)
Also good choices could be, but those already have sort of leaderheads made by fans:
the Hebrew (i use the atlantean leaderhead made by piernik to play them in my games)
the Poles (also by Piernik an excellent leaderhead that i use)
the khmer (i put them in my games by using the early animated bald version of the korean leaderhead i got by somehow)
the inuit (there is a leaderhead for them but I think it isn't as good as the original ones)
the mali (also a not as good as the original leaderheads is made for them)
and the ethiopians (a very good leaderhead for them is already made by sween)
If the portuguese aren't in C3:C i'll use Mursilis. He at least looks Portuguese.
If the Dutch aren't in. I'm just gonna be depressed
But this all is just my choice. I'm curious what the atari/infogrames choices are.
I was greatly amazed by the Sumerians and Hittites for instance.