Gangbuster Thorul, somewhere on the Internet is/was a fanfiction site that had as its main premise that the Five Worlds are still in the Netherverse patiently waiting for their heroes (and villains) to return from that epic battle at the Dawn of Time. By now most of the secret identities have been uncovered -- it didn't take long for those investigating the missing Clark Kent to find evidence linking him to Superman, and Alfred finally told all after a year of silent soul-searching.
A fascinating idea, but I don't know if this was seriously pursued by the original fanfiction writers. One might imagine Supergirl's Gold-Kryptonite depowered clone having a daughter, which would be half-Kryptonian and just now developing superpowers in her mid-teens. (Gold-Kryptonite's effects were apparently not passed down to the next generation.) This new Supergirl would then be the first Post-Crisis "New Gen" superheroine in the Netherverse, and possibly the first able to travel between the Five Earths. A lot of potential stories there...
I haven't heard of that exact fanfic premise. I did however hear of this one:
Five Earths Project-
http://www.geocities.com/the5earths/home.htmAs to Kara's Gold-K depowered clone in 5 Earths Project, the powers were returned to her after Lesla Lar merged with the clone. Lesla became a hero and due to the wierdness of her powers, she has energy powers on Rokyn and is th epremire hero of Rokyn known as Valor. Lydia-7 is now the new Supergirl of Earth 1.
At any rate, as long as the universe of the Nail is existant, the multiverse does exist in some way.