Carnage or Rhino; I'd like it. The problem lies, the third film included Sandman and Venom, but there was no reason for Venom to be in the film, there really wasn't. It was only fan service, I believe. They should've built Eddie Brock up during the prior films and then, strike with Venom in the third. It was too soon, and I didn't find myself caring for him at all. Sandman was cool, and New Goblin was really the only sensible and logical villain in the third one, even though he turned good in the end.
I'd like to see Rhino in the fourth one; Carnage, I love the character, but there needs to be a reason for him to be there, not some incoherent mess like Venom appeared to be. Rhino AND Lizard would be perfect villains for the fourth; they can use the Iraq War for Lizard's storyline, where he loses his arm and such, and Rhino just OWNS. Then again, She-Venom could be a contender, as can the Hobgoblin.