The thing is this: when it comes to guessing the new civs likely and wanted are two different things. Objectively, I'd say likely are returning favorites with some geographic balance.
I had originally guessed Khazars, Ethiopia, Zulu and Sioux. But the choice of Huns over Khazars, while silly, has been revealed. In a game with a religious importance and given that Africa is underrepresented by far, Ethiopia is probably the most likely.
We've already got several returning favourites (Netherlands, Maya), one or two returning couldn't-care-less like the Celts, and one new non-civ masquerading as a civ (Huns). I still maintain that the only significantly-sized area of the globe without Civ representation is Indonesia - surely it's at least as deserving as Polynesia. Yes, I think it likely they'll try to represent every continent (or at least region, "The Americas" likely counting as a single region). That would also give them an excuse to insert new architecture graphics which didn't make it into the main game, and in an expansion whose new graphics would otherwise be limited to a couple of dozen unit icons and the leaderheads, that could be seen as a draw. I think we'll probably see at least one additional Civ that's been in past games, but hope for at least one more that's all-new to the series.
On a slight tangent, what about city-states people would like to see? With more CS types and possibly CS-specific resources (like Tyre's jewellery), they may be trying to give the CSes individual personalities - there's no need to add many more numerically since the existing ones can't all fit into one game (I think there are more than 18?). It bothers me slightly that so many of the CSes aren't really CSes but are capitals of nations not represented in the game, while some genuine CSes are absent. Examples:
Zanzibar (commercial, unique resource: slaves?. Starting location tends to favour access to spices/ivory)
Crete (religious or cultured, representing the Minoans)
Troy (militaristic)
Syracuse (militaristic. More likely to gift siege weapons/Great Engineers than other units or GPs?)
Zulu doesn't fit the theme as well, however they are a favorite and on 2kgames radar at least a bit (the april fools I think may have had one or two actual reveals slipped in).
I thought that turned out to have actually been invented by CivFanatics staff, not to be a 2K April Fool's? If so, I wouldn't expect them to have foreknowledge of what's in or out beyond what's already been revealed.
And I'll say it again: the details we've been given so far are to generate interest and publicity. If the Zulus were in, chances are they'd have been top of the list for an early preview along with the Maya and the Netherlands - after all, who's ever asked for a Hun civilization? Does anyone much care about having the Celts as a civ?