I'd rather they never touch Carnage to be honest. He's generally far too one dimensional and was a poor substitute for Venom who started out as a pretty decent villain and then was kind of ruined by trying to shoe horn him into an anti-hero role.

For what it's worth, I believe the talk was that the second villain might be the Proto-Goblin, though looking at the cast list on Wikipedia that seems less likely now as the actor thought to be cast as his alter-ego is playing a different character. Mind you, I suppose that doesn't make it impossible, but I'd rather not have a villain that's basically a half-assed goblin character. Better to leave them alone and just re-do the Green Goblin in a movie or two if you want to go that route.