I'd like to see a Koala, a Hermit Crab, a Scarecrow (see below), and a Corn-based Pokemon.
The Koala could be Grass-type (or Grass/Normal, just to mix things up), the Hermit Crab could be Water, or perhaps Water/Steel (for the protective shell), or Water/Ground (since they can survive on land), the Scarecrow could be pure Dark (to differentiate it from Cacturne), and the Corn-based Pokemon could be Grass.
I'd like to see some cornfields in the next generation games, if you couldn't tell. I think it'd be a cool idea. *shrugs*
Also...is there a plant that can survive in extremely hot climates/conditions, like near a volcano? I think a Fire/Grass type would be cool, but I don't know if there's a plant (or animal, I guess) that it could be based off of.