Theory for Green Goblins Return in SM4
Now, a lot of people who are speculating on which villain from SM3 will come back are counting out Harry just because we saw him get buried and "die".
I was researching through some old Green Goblin stuff I have and came across the comics showing the first Green Goblins "resurrection" after Norman is killed.
Norman GG is killed by his own Glider ala Spider Man 1, but comes back to life because, while buried under the ground, his regenerative Goblin powers are still active. So he basically regenerates, breaks out of his coffin, and resumes his attempts at killing Spiderman.
Why can't this happen to Harry in Spiderman 4?
it's already been set-up perfectly, Harry is impaled by his own glider and "dies". He is buried in a cemetery and forgotten. It would make perfect sense for him to regenerate and come back to life, true to the Comic Book (Well, almost. Just replace Norman with Harry 😛 ). It would also provide an interesting twist in the movie that a lot of people didn't see coming.
Or hell, even if that is scrapped because Harry and Peter have already bonded and the producers wouldn't want to mess that up, they can just have Norman come back to life, which would fit in with the Comics perfectly.