Earth-Zero is the New 52 Earth, "ground zero" for new readers that are put off by decades of continuity. The titles set on this Earth could be done as finite seasons like Marvel's doing, making the entire family of titles more appealing for new fans.
Earth-Zero titles would have featured Superman: Man of Steel, Batman: The Dark Knight, The Flash, Green Lantern, and Justice League. Using titles similar to movies would make these books more recognizable to non-comic readers and differentiate them from the books that would make up the "core" DCU.
Earth-One would continue to be explored in OGNs, as that format has done very well for DC.
Earth-Two would be home to Golden Age titles like Justice Society and World's Finest.
Earth-Three would be home to a title simply called Injustice, dealing with Lex Luthor's never-ending battle against the evil of the Crime Syndicate.
Earth-Four would be home to the pre-Flashpoint, "core" DCU. This is where the classic iterations of the DCU would live on. Here, titles would include Superman, Action, TEC, and Batman, all returned to their original, pre-Flashpoint numbering. Other titles would include Hal Jordan: Green Lantern (or HJ and the Green Lantern Corps), Wally West: Flash, JLA, JSA, DC Comics Presents, Brave And The Bold, and Super and Bat family titles.
Earth-Five would be home to Captain Marvel and his adventures and eventual Marvel family.
And so forth. Books in each family of titles would bear a small logo in the masthead indicating the Earth the book is set on. The inner cover of every comic would contain a simple legend denoting the designations of the Earths and the titles set on each.
Earths outside of the core 6 could explore specific genres; one Earth would be home to Vertigo, another sci-fi titles like Adam Strange and Challengers of The Unknown, and even Earths featuring movie/TV versions of characters.