So far, there are only ten main characters and ten villain counter-parts, plus the possibility of hidden characters (assuming that the villains themselves are not hidden characters). Aside from the confirmed characters, the Cloud of Darkness, Golbez, Exdeath, and Kefka have all been spotted in a promotional poster. It can also be pretty much assumed that Luneth, Cecil, Bartz, Terra, and Cloud will all be in the game (seeing how they are the main characters of their games).
But there is no mention or even any suggestion that any character from a game beyond FFX will be included. So, I think it's fairly obvious that Square Enix plans to only use character's from games one through ten, meaning that it's extremely unlikely that we'll see Vaan (FFXII) or Lightning (FFXIII) in the game.
@Droginator - I don't see why there shouldn't be any FF3 characters. Thanks to the DS remake, the characters got a solid image and personality, and supposedly you are able to change Jobs (i.e. fighting style) in Dissidia, so they should be able to come up with plenty of possible character designs for the FF3 characters.