I'm guessing Chespin will becomce Grass/Ground or Grass/Poison as it's foreign names use chestnut's and hedgehog's and also chinkapin is a chestnut.
Fennekin might become Fire/Flying because it's big ears are the largest part, but the tail and face could lead to other theories. The hindlegs however look like they have wings on top or something.
And finally, I think Froakie will become either Water/Ice or Water/Flying because of the cloud like thing around it's neck, and it's other names include things like foam and bubbles.