I'm really hoping for Grass/Rock on Chespin. I can see it being almost like a knight, with maybe big shoulders, long claws, and a tail with a ball with spikes. Keeping the hat though, with a horn.
As for Fennekin, I think it should be very feminine. Fire/Psychic would be best IMO. Hey, if Chespin is going to be knight like, why not make a "royal" theme with the starters? Anyway, I'm hoping that the fire hair comes out of it's ears and makes a necklace with a orb on it. With red markings on it's legs and two tails dipped with red. Keeping the tan coloring of the body.
This may suprise people, but I'm really, really, hoping for Water/Dragon on Froakie. It could be a king with fin like dragon things on it's head making a crown (Think of Dragonite's Wings, but smaller I guess). Then, a moustache with Dragon Whiskers, like Magikarp kind of. Huge cloud as as a tail (think Whimsicott maybe?), along with clouds around it's wrists. Can learn fly and floats on clouds. Has sharp edges on arms, a bit like Gabite, but thinner with two on each.