I actually think it would be really cool if it where Grass/Dark, but I'm not so sure that's the case. Not because of it's happy looks, but something more, I'm looking at it's moveset.
Fennekin/Braxien's moveset contains Fire/Psychic attacks. From this, we can tell it's final evo will be Fire/Psychic, like everyone predicted. (Psychic, Glow Punch, Psybeam)
However, Chespin/Quilladin's moveset contains no Dark type moves. It's non-grass/normal moves include Rollout, Aerial Ace, Pin Missle, and Mud Shot. Aerial Ace/Pin Missle fits with it's spiky theme, and Rollout looks like a reference to Quilladin being a ball, because many Pokemon have moves based on their appearance. But what about Mud Shot? This is the move that leads me to beileve Chespin's final evo will be Grass/Ground. It's typing even fits in with it's coloring, which is green and brown, and sort of makes it resemble plants growing in soil. I would've really like Grass/Dark though, seriously, and maybe Gamefreak will surprise us.
