Despite its massive box office, $336,530,303 in the U.S. alone, Spider-Man 3 was arguably the biggest let down of the Sam Raimi directed trilogy, with too many villains and new characters for Raimi to handle. Raimi has an explanation for this, which he shared with Empire Magazine in its latest issue:
They really gave me a tremendous amount of control on the first two films, actually. But then there were different opinions on the third film and I didn’t really have creative control, so to speak.
Raimi goes on to say that this trend won't continue in Spider-Man 4:
The best way for me to move forward on films, I realize…and this was a
lesson I had to learn for myself…is that I’ve gotta be the singular
voice that makes the creative choices on the film.
There's still no word yet on who the villain might be, though rumors have mentioned both Carnage and Electro together, and Morbius the Vampire separately. Carnage would be an interesting choice, considering his character's origin being so close to Venom's, the Topher Grace character that Raimi was forced to use in Spider-Man 3. Regarding Venom, Raimi said:
I don’t even want to comment on Venom, because I know he’s a great
character and all the fans love him. I never want to say anything bad
about a much-beloved character because usually it turns out that I’m
the one that doesn’t understand what makes it great.