Sam Raimi, promoting Spider-Man 2 on DVD (which is out today) told USA Today there are a range of villains that may be in the third film including the Green Goblin - possibly resurrected by Harry Osborn (James Franco) - the Vulture and Kraven the Hunter.
"I'm trying to figure out what (Peter Parker) is going to be learning on this next adventure," Raimi says. "Once I know this journey and the conflicts to put in, I would choose the villain that best represents those conflicts."
Raimi is working on the screenplay with Alvin Sargent, who worked on the second film.