Well let's see. Spiderman 2 sets the stage for 3 villains, The Lizard (Curt Connors), the second Green Goblin (Harry Osborne), and Manwolf (John Jameson). But Manwolf is one of the most hated comic book villains of all time, so I think we can throw him straight out. The Lizard seems to be unlikely as well, because Doctor Connors now teaches physics, rather than having knowledge in the biology that would lead to his transformation. Like John Jameson, I think his character was placed in the movie just to add more grounding in the comic book world. Something is going to be happening with Harry, that is clear, but it isn't clear whether the revelation that his father was the Green Goblin will horrify him and cause him to forgive Peter, or whether he will continue with his lust for revenge and become the Green Goblin. Both scenarios seem equally likely.
Venom I suppose is a possibility since they could have John Jameson bring it back from space (that's pretty much how they did it in the show, if not in the comic, after all), but considering that Eddie Brock has not yet been introduced, much less his reason for hating Peter Parker, that seems like a lot of development to occur in one movie, especially considering that the development with Harry needs to occur. So venom to me seems unlikely in the next movie. Perhaps if they end up doing 4 (which probably won't happen, but you never know) they will have him in the fourth, but for the third it is very unlikely.
So what other villains could we have? The more classic ones would be Shocker, Hobgoblin, Rhino, Mysterio, Scorpion, Chameleon, Vulture, Beetle, Kraven the Hunter, Electro, Sandman and Kingpin. That includes all the members of both Sinister Sixes and the Sinister Seven who have not appeared yet, among other things. Hobgoblin is too similar to the Green Goblin, and if they were going in that direction they'd use Harry as the second Green Goblin. Shocker, Rhino, Beetle are not teribly interesting to me, and Sandman is rather unknown outside of the comic books. Kingpin was used in the Daredevil movie, so that's a no go. Mysterio and the Chameleon always had ambiguous motives, so I don't know how well they would work in these movies that seem to like human villains. And I'm not quite sure what a struggle with Electro would cause Paker to realize about himself, since Electro was just a guy who gained the power to whip electricity around. So that leaves us with Scorpion, the Vulture, Kraven the Hunter, and another Green Goblin any of which would be interesting villains if you ask me. It all depends on what conflict the next movie is supposed to be about, really.