Given that one of the new world wonders is the Maoi Statues, I have a sneaking suspicion that the Polynesians will be included. Certainly, a cultural and geographic group that has not yet been covered in the game's history.
I agree with snipper re: Cristo Reductor. Given that the Dutch (a colonial power) is given as a "civilization," I think it makes more sense that Brazil will be lumped into the Portuguese "civilization." Also, are we sure that the Sioux are the own distinct civ? The site seems to indicate that there is a general Native American civ, with Sitting Bull (a Sioux) as one of the leaders. I know Sid goes back and forth between the Sioux and the Iroquois... maybe the game developers just lumped them together? Kind of a cop-out, I think.
As for more speculative civs, my guess is probably a southeast Asian civ (Thai, Khmer, etc.), Maya, Hebrews, Byzantines. It is difficult to surmise, because the game is suppose to cover the latter part of history, yet the confirmed list shows that even ancient civs (read: Babylon) are included.