As much as I loved Molina as Ock in Spider-man 2, and as much as I would love a return of him...it really shouldn't happen.
First, it would be like jumping the shark- bringing him back this early....it would be admitting that they have nothing left to offer and are going to be "safe" and bring back something they know worked.
Secondly, he is dead. He swam with the SUN. That water was so hot that it would have evaporated his skin soon after he was in it. Also, his arms weighed about 100-200lbs. They also do not float.... The lights on the arms, blinked and went off at the end of his last scene....he is dead.
Thirdly, besides being more dead than a door nail....he snapped back to his senses and converted to the good side. He wouldn't be coming back as a "bad guy"...to do so would be an odd step.
The coolest way to bring him back- would to have been to have the "Master Planner" appear in voice only in Spider-man 4...orchestrating the events of the film and the film's villains...to be revealed as Ock in Spider-man 5...but that would be very hard to achieve given the reasons that he is dead and, if miraculously alive, a good guy.
-R