I had a bit of a thought a while back. All the previous Grass starters (except for Snivy) ended up being dinosaurs. However, Chespin is clearly a mammal. I suspect it may end up evolving into some sort of prehistoric mammal of sorts, perhaps Diprotodon (a name that literally means "two forward teeth"), as it bears a little resemblance. The other two, well, I'm not entirely sure. It's odd that Froakie is clearly a fully formed frog; perhaps it evolve into a giant tadpole (borrowing some inspiration from the Paradoxical Frog)? Probably not likely, but it would be cool. Fennekin... well, it's hard to see where that one could go other than a Psychic Ninetales clone or another Fire/Fighting...
3DS FC: 3024-6140-6327 (please let me know if you add me, so I can add you too)
Friend Safari is Fire with Pansear, Larvesta, and Braixen.
Do the impossible and kick reason to the curb!
Given the choice of one evolution line to remove from existence, I would look at Garbodor and Vaniluxe, smile, and remove Arceus.