Juggling multiple incarnations of characters across multiple Earths is nothing new for comic book fans, and it seems comic book history can provide a model for how the movie multiverse could work?
DC’s multiverse officially started in 1961, as The Flash No. 123 introduced Barry Allen’s speedster to Jay Garrick, the Flash that appeared in comics published during World War II. The explanation given was that Barry existed on Earth-1 — the name debuted in 1963’s Justice League of America No. 21, where the story was called “Crisis on Earth-One,” fittingly enough — while Jay and his fellow old-school heroes lived on Earth-2, a concept explained away with fittingly ridiculous pseudo-science: “My theory is, both Earths were created at the same time in two quite similar universes! They vibrate differently — which keeps them apart!” explained an excited Barry Allen. “Life, customs — even languages — evolved on your Earth almost exactly as they did on my Earth!”
The concept of the multiverse proved to be a favorite of both fans and creators, with the Justice League of America series having annual storylines where Earths-1 and -2 would crossover, with more and more Earths being added each time: Earth-3 was an Earth where morality was inverted, and the Justice League of that world was called the Crime Syndicate. Earth-X was an Earth where the Nazis had won World War II, and so on. The multiverse kept growing and growing for the next two decades.
As this was happening, superhero comics in general were becoming a more formal, more disciplined genre — thanks, in part, to the success of Marvel’s approach to storytelling, where everything was connected and part of one particular tapestry. Contradictory approaches to characters, and belief that audiences aged out of the material and were replaced by new readers every handful of years, were abandoned, leading to the creation of even more alternate Earths through necessity: writers or editors who allowed continuity errors to appear in their stories would find their work shunted to new Earths — “Earth-B” was the unofficial name for stories written by Bob Haney and E. Nelson Bridwell, both of whom wrote with an enjoyable looseness for previously established facts, for example.
Despite this, however, almost every superhero title published by DC took place on Earth-1. Sure, there were outliers — 1980s series All-Star Squadron and Infinity, Inc. were long-running series that were set on Earth-2, and animated spin-off Super Friends existed on Earth-1A, just to be complicated — but the majority of stories were set in the same shared world of Earth-1, with the other worlds being used sparingly. After the conclusion of the 1985/86 series Crisis on Infinite Earths, they were abandoned entirely for more than two decades.
The reason for this was, simply, that the multiverse proved to be a confusing topic for fans. “Which story took place on which Earth?” became an increasingly common refrain in letters to the editor, with other fans wanting to know which stories “counted” — the implication being that only stories that happened on the central world of Earth-1 mattered in the grand scheme of things. Related to that, there was an unstated shared belief that stories set on other Earths — even something like All-Star Squadron, which ran for five years — were either throwaway filler or intended for a niche audience, which automatically restricted their reach. Eventually, it became easier and more practical to redraw the boundaries of the entire fictional universe and set everything on the same Earth.
(This would be undone to an extent in 2006’s 52 series, which restored the multiverse in a far more limited fashion, with its use even more limited in subsequent years.)
If Warner Bros. is to fully explore the cinematic potential of a multiverse, it would make sense that the studio will follow the lessons of DC’s comic book output, with the “Earth-1” of movies such as Justice League, Wonder Woman, and Aquaman will remain the primary focus of the studio, with the “Earth-2” of Robert Pattinson’s The Batman and any future movies being essentially standalone in nature with the framework of the multiverse (and the Flash crossing between worlds in his own movie) teasing the idea that, in some grand sense, it “counts” as part of the larger story.
To do anything else — to try and expand multiple franchises on multiple Earths simultaneously — would be folly, arguably overestimating both the audience’s interest in, and focus on, the minutiae of its cinematic mythology… and surely the very existence of Zack Snyder’s Justice League is already ticking that particular box elsewhere.