I think it's safe to assume that Black Legion, the poster-boys for Chaos, will be there. A Word Bearer is shown in some concept art and I believe an Iron Warrior was shown briefly the Meet the Dev Team video. While none of this is set in stone, most people are assuming that these three legions will be in, which the leaves the 4th legion up for grabs. The three previous legions are all Undivided, meaning that they generally worship all the Chaos Gods equally. Following this trend, we can probably assume that the 4th legion will also be undivided. Of the "big nine" legions for Chaos, that leaves two possible legions: Alpha Legion and the Night Lords. There's also talk of a 5th legion being added in over the course of the next two years. Assuming that it could be any legion, who would you want?