As I already pointed out in another topic, Water/Flying is not weak to Grass, so Chespin must be Grass/Rock or Grass/Electric to compensate... and Grass/Rock is not weak to Fire, so if it's Grass/Rock, Fennekin's type is tied too, and Fire/Psychic is not on the list (Fire/Fighting, however, sadly is.)
I don't understand this rationale. Empoleon isn't weak to Grass but that didn't stop them.
Granted, it is weak to Torterra's secondary type but you're not taking secondary types into account anyway.
Clearly, it's not paramount that all fully-evolved starters remain weak to their triangle counterparts. They do, however, seem to always be weak to another starter if you take both types into account (and sometimes they're weak to both other members of the triangle--Empoleon isn't weak to Grass but is weak to Infernape's Fighting and Torterra's Ground).
So basically they can be any type they want as long as they can still take SE damage from 1 or both other starters. If Froakie became Water/Flying it wouldn't matter because if Chespin became Grass/Rock, it could still use Rock attacks to hit it with SE damage, just like how Torterra could still beat Empoleon. However, I don't predict either of those types happening.
Or GameFreak can decide that they can be whatever type because the lesson of the Fire/Grass/Water trio is already sunken in by the time they're all fully-evolved, given how often you face your rival(s) when your starter is in its first and second stages.