Fan Casting Captain Atom/Nathaniel Adam in Superman 3: Public Enemy

Seemingly dies whilst saving the world by obliterating a kryptonite meteor, however much smaller chunks survive which Luthor salvages (this is in addition to the kryptonite that fell to earth sometime earlier which is what prompts Luthor to monitor the larger meteor) Has actually transported to the future, is approached by a member of the Legion Of Superheroes in a mid credits scene. Returns just before Luthor is about to kill Superman and saves him. Returns via Legion flight ring. Story would be explained in Legion Of Superheroes movie.