With "Superman" set to debut in July 2025 as the first live-action film in James Gunn and Peter Safran's rebooted DCU, attention is increasingly turning to how the entire universe will converge and integrate in the future. 
In a recent interview with Screen Rant, Gunn revealed that the various projects in Chapter One: "Gods and Monsters" will converge into a "major event," though he emphasized that "it's not the kind of event people are imagining."
Fans quickly began speculating. Among them, a theory proposed by user @BobbensArtsDen garnered significant attention, suggesting that the major event Gunn is setting up could be related to the famous "Earth-Three" concept from DC Comics.
This universe first appeared in 1964's "Justice League of America" #29, created by comic artists Gardner Fox and Mike Sekowsky. It's a parallel Earth where all the superheroes are villains, and the only truly heroic figure is its Lex Luthor.
This theory is not without basis; in the "Superman" film, David Corenswet not only plays the main universe's Clark Kent but also appears in a post-credits scene as Ultraman.
Further Reading:
Who is Ultraman in DC's "Superman" Film? Does He Snort Kryptonite? The Evil Superman? An In-Depth Look at Ultraman
Additionally, in the upcoming "Peacemaker" Season 2 trailer, John Cena's Christopher Smith, aka Peacemaker, will also confront a doppelgänger version of himself, further hinting at the introduction of parallel universes. 
If Earth-Three indeed becomes a central storyline, it would not only signify the debut of the Crime Syndicate of America but also potentially introduce characters like Superwoman, Johnny Quick, Atomica, and Sea King.
via： Comicbook.com