What Could Have Been: The Kingdom was originally planned to be a prequel which takes place between the "present" main continuity and the "future" in Kingdom Come. Another source say The Kingdom would reveal the Kingdom Come universe to be the post-Crisis version of Earth-Two, which was altered rather than erased from existence.
If it's true, then they changed their minds so late that they couldn't completely remove that plot thread from the story as The Kingdom is bookended with Superman of Earth-2 pounding on the walls of reality.
