Originally Posted by Fat Cobraroll
I think Fire/Ground can be ruled out for a starter, as this would give it an unusually severe disadvantage against a Water starter, and by extension, the inevitable "Water Pokémon everywhere"-routes. It would also nullify its Grass resistance, causing some confusion (admittedly, not as bad seeing as Empoleon already did this, but still...).
The starter Pokémon is almost always a fairly balanced one, with some advantages at several points in the game, but rarely a severe disadvantage, at least not early on. Game design-wise, it's meant to be a reliable 'mon and the team's powerhouse for a while, designed to stick with you from the start of the game to the end. They wouldn't give it a 4x weakness against an extremely common in-game type. True enough, Charizard was 4x weak against Rock, but in that game there were only two Rock type moves, neither of them very common, and Charizard didn't gain this weakness until post-all-the-caves-game, where you had a team to make up for its shortcomings. And since the very first game, if my memory serves me correctly, we haven't had any starters with a double weakness.
Similarly, unfortunately, Grass/Dark would give Chespin a disadvantage against the common Bug type. This could be mitigated somewhat by not giving it a Dark type until its final stage, at which point you won't find many strong Bug types (Bug is an early game type, as opposed to stuff like Dragon and Ice). Still, the presence of the dreaded double weakness is a strong argument against Chespin's possible Grass/Dark type. Grass/Poison, Grass/Ground or even Grass/Rock could work. Grass/Steel is out of the question.
We could have a secondary type that nullifies the primary weakness or resistance (See Empoleon) or hit the rival starter super-effectively (Torterra, Infernape), but I think GameFreak want to keep double weaknesses to a minimum, to keep your precious starter a solid choice throughout your entire adventure, and prevent it from being a total liability at some points in the game.
Torterra was 4x weak to Ice and Swampert was 4x weak to Grass. Granted, that was Swampert's only weakness, but Torterra still got shafted pretty hard. The Sinnoh starters are weird because in their fully evolved forms they were all super effective against each other (Empoleon needed Ice Beam, of course, but most people are going to have that on him anyway). Gen II and V were the only ones without a starter that had a double weakness; it actually looks more likely that there will be one in gen VI. It's nice to be hopeful that every one of our new starters will be great, but there's still a chance we'll end up with another Meganium or something. (I fucking love Meganium, it's one of my favorites, but come on guys it's one of the shittiest fully evolved Pokemon there are.)
Yeah, I know this has been said a million times, but I seriously hope Fennekin doesn't turn into another Fire / Fighting. I'm really worried about this. We need some innovative starter types, like Grass / Flying or Fire / Psychic. That would be so cool.
Chespin being Grass / Dark would actually be pretty nice. Like a Shiftry that didn't suck.