In an interview with The BBC, Sam Raimi confirmed he has finished the basic story outline for a third "Spider-Man" film and spoke a bit about how the events in the second one will affect the Harry Osborn/Peter Parker relationship:

"It's going to follow the natural progression of the growth of these two individuals, Mary Jane Watson and Peter Parker. It will be a love story and it will track the relationship as they encounter new conflicts that separate them. And hopefully their own will and love for each other will bring them back together in the piece.

Harry will be a problem, but I'm not certain if Harry will become the Goblin or not. The thing I'm really not certain about is if he comes to feel, "Now I have the means to destroy Spider-Man - I was too weak before but now I have a way and I will become the man that my father always dreamed I could be." Or, does he feel: "Now I understand why Spider-Man had to destroy him, because he was a mass murderer and now my anger can drain away from me." It could be an answer to a question and it ends the pursuit, but I'm not certain which way to go".

Later he chatted with French site DVD Rama more about the potential villains:

"I have not decided the vilain of Spiderman 3 yet. I have to talk about it with Avi (Arad) & Laura (Ziskin), our other producer. We are currently working on several tracks. The problem is to be in Peter's mind at the end of Spiderman 2: What does he want in his life? What problems will he have to struggle with, as an adult? THEN we will find the best villain to show these conflicts".

In the same interview, Avi Arad and Sam Raimi confessed they both like Blackcat. They talked a lot about her for "Spiderman 2", but it was not the right time to use this character in the story

Thanks to 'Gustavo' & 'Antoine'.