Alright, it's time for another speculation theory!
Even if I'm pretty sure that Froakie is already confirmed to obtain the fighting type during the evolution, I wanted to put into effect the very first impression I had about it anyways 8D.
Giulio-91 and I were discussing teh newly revealed starters, and we came up with the idea of Froakie's evolutions being flying-typed. And if you think about it, everything matches: the French name "Grenousse" is made up of the words "grenouille", which means "frog", and "mousse", which means foam. Foam is actually a very important element, since it is, in terms of chemestry, a mixture of gas and liquid. In other words, flying and water. In addition, the German name "Froxy" contains a very suspicious "oxy", which could remind of "oxygen" which is, again, connected to the flying type.
But again, these are just speculations. I really hope that it'll become a flying frog of some sort, but I highly doubt it ):
Do you like my rendition of it? I tried a vaguely Sugimori-inspired style..but hey, it's just an attempt (: