I thought Bryan Singer was in the process of scripting a sequel to Returns, and I remember an interview with him where he admitted the faults of Returns but mentioned that now with things in place the sequel would be the Superman 2 of the franchise with super villians/battles galore. I was kind of looking forward to it. I may be in the minority, but I like how Returns fits in with Donner's Superman, and that's no accident either since Singer was given an advance look at the Donner Cut of Superman 2.
As far as Donner, if you watch the extras in the Donner Cut of Superman 2(which is AWESOME, btw), you get the impression that he's semi-retired. I also remember reading/hearing that after the ugliness of him being removed from Superman by the $alkind$ that he didn't want to have anything to do with the film again. It took 20 years until he looked at working on the Donner cut, and at rabid fan insistence that he do so. In some of the clips, it seems kind of obvious that he really hasn't looked at the flick at all since then.