Should we assume that the Pokemon shown in the trailers will be obtainable in X and/or Y? If that's the case then we know that, at least, these past Pokemon will be available.
Pikachu (Pichu, Raichu)
Kirlia (Ralts, Gardevoir, Gallade)
Dratini (Dragonair, Dragonite)
Machamp (Machop, Machoke)