With the civs revealed so far, we have three with a European culture (Dutch, Celts, Byzantines), one with a Middle-Eastern/North-African culture (Carthage) and one with a Mesoamerican culture (Mayans). As Firaxis seem to have been trying to have a more balanced selection of civilisations from around the world this time round I'd be surprised if they included any more from Europe with this expansion. In my opinion, I think the most likely possible candidates might be:
Ethiopia: Perhaps the most likely, in my opinion, as Africa is currently seriously under-represented, and could also do with a civ that isn't military-focused. They'd also fit in really well with the re-introduction of religions.
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Zulu: Although they only formed a very short lived state with only local importance, they are a 'classic' civ, having been around since the start of the series. It would be strange not to see them included. Zimbabwe may make an interesting alternative though.
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Khmer: A lot of people have discounted the Khmer already by saying that they're too close/similar to Siam, geographically and culturally. However, really they're as different in culture as, say, France and Germany, and were at one time the dominant superpower in Southeast Asia - I'd personally be really disappointed if they weren't included.
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Sioux: If Firaxis decide to include another 'Native American' civ, the Sioux may be the most likely choice as they're fairly well known, and have already appeared once before in Civ II (Also, Sitting Bull was the leader of the Native Americans in Civ IV). Possible alternatives include: the Apache/Comanche, the Pueblo and the Mississippians.
Sumeria: Another 'classic' civ - having appeared in both Civ III and IV. Could represent the Middle East.
I'd also be amazed if Portugal wasn't released as one of the first DLC after the expansion, perhaps in a double pack with Kongo? Poland, Austria-Hungary, Sweden, Italy, the Hittites, Phoenicia, Israel, the Moors/Morocco, Nubia, the Chola, the Kushan Empire, Vietnam, Tibet and Majapahit are also possibilities.
I apologise for the long post. I really wanted to get that off my chest.