That Eevee idea sounds great, and I'd be all for it. Of course, character transformations are sort of...not a thing anymore, I guess. Though Super Smash Land has Vaporeon as an unlockable character if you wanna try that.
For this version they're not a thing, but next time, who knows. The real thing would be whether they could design Eevee and possibly each of it's evolutions to be decent and not give us 9 characters in one whom all suck like Pokemon Trainer.
I was kind of hoping this one would have had Eevee playable, given that we had a new Eeveelution in gen 6 and they seemed to really push that for awhile. I'm extremely happy that Eevee still made it in via Pokeball though, but a playable one would make me happy forever. :3 Definitely would be interesting in seeing how Eevee would operate, though!
I assumed we would be seeing Mewtwo for sure too once Charizard was confirmed, but nope. Mewtwo and Charizard were the only ones to get two Megas in XY as well, so it's quite surprising (and disappointing) that Mewtwo didn't make it in.

