I'm that guy who spends months trying to figure out.
Overall, I like Water-type starters. I picked Squirtle, Totodile and Piplup, and would've taken Froakie if Squirtle wouldn't be available as well. Mudkip is cute, I liek it, but I just have a bad feeling about him that I can't describe.
In my first Generation III run (Sapphire), I took Torchic because it was the cutest thing ever and I knew it evolved into something badass. But he is too popular and I'm not really into Fighting types.
Treecko is the one of best Grass starters in my opinion, but I just don't like Grass starters so that kind of takes it down.
Yeah, this is going to take a while.
EDIT: Wait a second, we don't even know what starter Pokémon will be in there! Maybe they'll introduce three whole new lines! No, I don't think so.