my initial thought was Rome, but it does occur to me that it's probably not as close to done as, say, the Aztecs (which we've seen, albeit very blurrily, in-game? though I guess that could mean anything.) Rome's been "leaked" but the only real look we've got is that not-really-animated shot of Trajan. Of course, that could have been filmed anytime, and I know very little about the timeline of these things, but i'm leaning towards Aztecs or India, both of which have been confirmed through dev interviews. i do think we're more likely to get more info on mechanics (religion, diplomacy) before we get looks at civs that are likely to feature those mechanics, which could be any, so who knows.