I actually don't think Zod belonged in Superman Returns. In Superman II, he was dropped to his death in the Fortress of Solitude. Even though Donner had him survive, nobody has seen this version, and would make Superman Returns even more confusing of a "semi-sequel", even though it does solve the problem of why did Superman skip out for 5 years.
I think in the end this whole sort-of a sequel to Superman II was basically to be able to skip the whole origin story, which would take up about half the film.