Someone from another site has an interesting theory.
"A bunch of people have been telling me that Froakie is themed after japanese thieves or bandits, that white thing isn't a cloud it's his loot sack (which could also simultaneously be a reference to those frogs who glue their eggs to themselves) That's also what those oddly phallic facebumps are, where he ties the cloth
So his evolutions are likely to continue the bandit theme
This does imply it'd be Dark rather than Fighting but I'm hoping it'd still work for the other." --- Note this, I am not Greek. Stupid name from years ago. GG7 is much better to call me.
I do think froakie is covered with foam, and not clouds, though I have been wrong in the past. However, I noticed the white bits on his nose look like tiny scholarly glasses, so maybe he could even be a psychic type down the road, not fighting. Oddly, the foam on Froakie is reminiscent of the foam nests that some frogs attach to themselves to transport their young in. I know it exists, but I've only ever seen it once or twice in real life going frog catching in swampy areas.
Chespin is a hedgehog...chestnut...thing. I can only hope it becomes good defensively, and gets many grassy spikes. It could be part bug, I suppose. But since it is based off of a mammal, it probably will not become bug in any way. X-scissor is a common move that can be learned by grass pokemon that have means of cutting via claws, sharp leaves, ect. Much like how many fire types can learn solar beam, and many water types can learn ice beam. (Samurott can learn X-scissor, it is a good physical move to use against grass types instead of largely special based ice moves)
Hm. Fennekin is a tough one. When something evolves it is easy to make a fire pokemon have longer and more luxurious fur as it evolves, much like flareon. But Fennec foxes are desert creatures so it may become a slender, elegant evolution since most desert creatures have sleek short fur that traps enough heat to stay warm, but in the case of the fennec fox, those large ears are also a means of cooling the body as well.
As all of the starters are slightly different, you never know if any of these three will evolve as a pure fire/grass/water type. Since it seems to be one of the most inconsistent things among starters so far. Gen 1 only had 1 pure type (Blastoise), Gen II had all three as single types, Gen III had the start of that weird fire/fighting trend and Sceptile was the only single type, Gen IV had ALL dual types (which I liked the most actually, as far as starter development across the games), and Gen V only had emboar as a dual type. Though it seems most common for grass types to be single type, while fire types have only been single type on one occasion.