In the Alex Ross art book Mythology, Ross talks about how he was planning to handle the sequel, The Kingdom. It would have revealed that the Kingdom Come world is not a possible future but rather a present day, resurrected Earth 2. But that idea obviously fell through after he left the project.

which rocks, because Kingdom Come rocked. can anybody else see Kal-L throwing on the red and black 'S'? the way everything played out for him in both Crisis tales, i'm more than sure.

which again makes perfect sense now, since Ross is now the 'Creative Consultant'. what better way to get that unsed idea out than through DC's first superteam?