So on the video that's been out for a while (below), the starters all use a move of their type against a Pokemon that it's super effective against (except Fennekin on Munchlax). Then, they use another move on another Pokemon- maybe hinting at their secondary types? I know lots of people have been hoping for a second relationship among the starters similar to the fire/water/grass one, but with something like psychic/dark/fighting or ghost/dark/fighting. Maybe Fennekin would be fire/dark, Froakie water/fighting and Chespin grass/ghost?
Chespin - Grass/Dark or Grass/Rock Fennekin - Fire/Psychic (I personally think this is the most likely; that move in the trailer definitely looked like a psychic move and there are few non-psychic Pokemon that just learn psychic moves like that) Froakie - Water/Fighting
I can see Chespin being Grass/Rock Fennekin strikes me as pure Fire all the way through...especially after three gens of Fire/Fighting -_- Froakie...Water/...something? It seems like it'd gain a secondary type, I just don't know what.
Old as time. We know what trollfreak are like. People have been craving a Dark/Psychic/Fighting type combo to the starters for a while now. Not saying it'll never happen. But don't think it's likely to happen.
My guess on the starters is that... Chespin will probably be either Grass/Steel, or Grass/Fighting or pure Grass. Fennekin will probably be either Fire/Psychic, Fire/Ground, or Pure Fire. Froakie will probably be either Water/Flying, Water/Electric, or Pure Water.
Sic vis pacem, para bellum. Don't make me HM01 you.