Nothing, Treecko is perfect. Honestly, I'd change Sceptile to be Grass/Dragon since its mega evolution is gone forever now. But, other than that, there's nothing to really change, Treecko is fine the way it is.
Sneasel, though, honestly... Make its evolution more accessible in the game its evolution debuted in. You can't get the Razor Claw until after you beat the damn game. Like, that's so stupid. Make it accessible at some point earlier than Route 224 in Diamond/Pearl!
Litwick, I wouldn't do anything. As it is fine the way it is. Learns pretty decent moves, is pretty easy to find in the game you encounter it in, etc.
And Dewott, I'd have Samurott Water/Fighting. "But, Kory, Pignite is--!" I don't care. I literally don't care. Samurott is disappointing.