Some time ago, I read that one of the original, never-used ideas for Superman during and after CRISIS ON INFINITE EARTHS was that the Silver/Bronze Age Earth-One Superman was to give his life to defeat the Anti-Monitor, and Kal-L of Earth-Two would survive to become the Superman of Post-COIE Earth. Kal-L would then reveal that his apparent old age was just a disguise all along (he removes his "old" makeup to reveal that he looks the same perpetual late-20s that he was during the Golden Age, and adopting the appearance of old age was just to better relate to his peers).
It's kind of a crazy cool idea that was never implemented, of course, but I forgot whose idea it was: Marv Wolfman? John Byrne? Does anybody remember where this idea came from?
Buried Alien (The Fastest Post Alive!)