Didn't Doc Ock redeem himself at the end of Spider-Man 2? Remember, he is not a villain..he's more of a tragic hero..someone who is ultimately good being controlled by his own flawed creation.
Wouldn't the kind of return your talking about completely ruin the thematic quality of his triumph over his own inadvertent evil? To me bringing Doc Ock back after SM2's ending by way of showing his survival would seem only like a cheap trick along the lines of Harry's amnesia from Spider-Man 3.
A better way might be to show other things Doc Ock did in Spider-Man 2 off screen. Perhaps along the lines of collaborating and forming a plan with Spider-Man 4's villain before the events of SM2 ended, and before SM4 began. Still however, this to me seems a little bit along the lines of the "Sandman killed Ben" plot line of SM3, which wasn't received well at all. It's probably better to leave Doc Ock out of future installments.
I don't mean to sorta judge your idea to harshly SMH12. I think it would work rather well with Doc Ock as he was portrayed in the comics (as a mad evil scientist), with the comic book logic surrounding character deaths (people don't die, they just go away for a bit).
I just feel that in the Spider-Man movies, the thematic undertones of these characters are more important then bringing them back, when more unique plot lines that take advantage of new and great characters are possible.
Maybe I just love Spider-Man 2 a bit to much.