The latest trailer for Pokémon X & Y reveals the ability to customize your trainer and certain types of Pokémon, the evolved forms of the new starters, and a new Mega Evolution for Mewtwo.
It turns out Mewtwo's Mega Evolution will differ depending on what version of Pokémon you decide to purchase. As you can see in the image above, Mewtwo will have different Mega Evolved forms in X & Y.
The trailer also reveals that your trainer can adopt different looks, as well as certain Pokémon like Furfrou.
The new starter Pokémon, Froakie, Chespin, and Fennekin's evolved forms also shown. The three turn into Frogadier, Quilladin, and Braixen respectively, which you will see in the trailer. Be sure to keep an out for Mega Garchomp and the new Pokémon, Meowstic, in the trailer as well.