The History of the DC Universe was to have been the final two issues of Crisis on Infinite Earths. It ultimately was published (separately), but the fact that DC history was in flux and would stay that way for years made it useless as an actual guide to the post-Crisis DC Universe.
Alex Ross's art book Mythology contained a rejected pitch he made (when he was still a teenager) to DC for Earth-10, a futuristic amalgamation of Earth-One and Earth-Two home to the Justice Union of Earth, which would've been part of the Crisis storyline. The team was an updated combination of the Justice League and the Justice Society with such likely updates as Green Bullet (Green Arrow), Johnny Nuclear (Johnny Thunder), and Galaxy Man (Starman).
One original idea for the ending would be the Superman of Earth-1 fighting the Anti-Monitor at the end and dying. Earth-2's Superman would then remove aging makeup, revealing that he actually had not visibly aged past adulthood. Then the original Superman would take the place as the "main" Superman again. However, this ending would have been inconsistent with John Byrne's Man of Steel.