Look closely at Froakie. Beyond just being a frog, he also has an odd white cloud thing behind him. I think this is one of two things:
A cloud that will allow him to fly upon evolution
A cape. As in a super hero cape.
The latter seems more likely to me, as it seems that the Fennekin might evolve into a psychic type and Chespin might evolve into a dark or ghost type as others have suggested. Froakie could complete the type triangle (although it is flawed) by becoming a fighting type. Plus, look at the gloves on his hands. Most pokemon don't wear those, but super heroes do.
I guess the gloves could also be aviator gloves.
I'd wager on one of these two typings. I don't see it as likely that he'll be anything but these or pure Water when he is fully evolved.