I have no idea how the legendaries will possibly be used in the game, but they still look amazing, so I don't really care lol.
Anyway, I think it would be pretty cool if they put into the game a thing where, like, you have to make certain choices, and these determine if you'll be good or evil (Sorta like Fable). Also, I think it would be cool if you could choose a sort of career and follow that with certain "missions" while working on the badges and elite 4 (sort of like Final Fantasy).
I know it seems a little abnormal for Pokemon, but I really think they should change around game play a little, just b/c it seems really repetitive.