Hm.. Poison seems to be the next thing to drop... But that's on the basis that it sucks offensively and that Weezing is as good as Poison will ever be defensively.
Basically what I think of poison type and Grass, Grass is combine with nearly ever type execpt Dragon, Normal, Ghost, Fire, and Steel and only one could work flavor wise.
Beg to question, why should we get rid of Steel? Because it won or almost won the typing poll? Not a good enoug reason.
Edit: To back up my shit on Grass and the remaining few type combo.
Normal/Grass: Okay what the fuck? Could anyway explain how that would look if that was a pokemon?
Grass/Dragon: The only one IMO that could work but having too many weakness irk me a tiny bit.
Grass/Steel: Blades of metallic grass? I don't think so.
Grass/Ghost: Could work but everyone and their grandmother already did something like this and it all involved pumpkins so resisting of creativity
Fire/Grass: Erh....right, flaming bush? Not buying it.