I like the idea of him eventually finding a way to pass the torch. Either he goes Superman Prime mode and lends his power to a universe full of his successors or Kingdom Come mode and becomes a figurehead for a new superhuman-human paradigm or Saga of the Supersons mode where he starts to ease his son into the role. I like Superman being a metaphor for the everyman and no one does the same thing forever. People change and retire and move on. I like the idea of Superman being immortal too much to have his story end in his death, so him ascending the throne as it were is a nice final page to his story.