If Joaquin Phoenix's Joker transitions to the wider DCEU, it should be via the multiverse, not Matt Reeves' The Batman finds itself in similar territory. Starring Robert Pattinson as the brand new Caped Crusader, The Batman is uncompromisingly violent, sitting firmly on the mature end of DC's movie output. Although the film looks more visually aggressive than Joker, The Batman is also a lone wolf with no franchise connections. For now, at least.
Joker was a massive success, and fans are chomping at the bat to see Joaquin Phoenix face off against Bruce Wayne. In-keeping with the overall anti-superhero ethos, however, Joker's ending doesn't exactly lend itself naturally to a sequel, and Todd Phillips has offered no sign that Pattinson's Gotham. While this is certainly viable from a thematic perspective, the timelines pose an impossible stumbling block. Joker takes place in the year 1981, whereas The Batman is set in the modern day. If Arthur Fleck is anywhere near Joaquin Phoenix's real age, he'd be a pensioner fighting Robert Pattinson's youthful Batman. If only a time-traveling superhero was bringing different eras of DC movies together under one umbrella...
Timeline gymnastics might make a meeting between Phoenix's Arthur Fleck and Pattinson's Bruce Wayne impossible, but 2022's Ben Affleck's DCEU Batman will feature when Barry Allen breaks the timeline, meaning the entire DC canon is apparently connected by the mysterious multiverse mechanic. If Michael Keaton can transition from Tim Burton's world to the modern day DCEU, Joaquin Phoenix's Joker can too, and this would open up a veritable treasure trove of dream confrontations. Phoenix vs. Battinson, Phoenix vs. Leto, even Phoenix's older Joker vs. Keaton's older Batman. The multiverse dynamic creates far more possibilities than simply setting The Batman in Joker's Gotham City.
Obviously, Flash and his Bat-companions wouldn't want to bring another version of Joker into the DCEU, especially one as demented and influential as Arthur Fleck. Nor do they have any business in the bleak world of Joker, which shares very little in common with the DCEU's world of metahumans, alien invasions, and ageless Amazonians. But when a time-traveler (usually Barry, let's be honest) meddles with the multiverse, once-separate worlds can merge, as seen recently in the Arrowverse TV franchise. The Flash could drop the mother of all post-credits sequences by revealing that the mainstream DCEU has folded into the Joker world... and a confused Arthur Fleck is released from Arkham Asylum only to find a completely different, unrecognizable world awaits him.
Given the massive age gap between Arthur Fleck and Bruce Wayne in Joker, Joaquin Phoenix is unlikely to ever clash fists against Batman in his own universe, making Flash the only person who can turn those dream battles into a reality. With that said, it's impossible to reconcile the idea of time travel and parallel universes existing even remotely near the world of Todd Phillips' Joker movie. Even more so than The Batman, Joker was so ruthlessly grounded that not even a hint of comic book fantasy would feel at home there, least of all a man using super-speed to break the barriers of time. However exciting Fleck's potential appearance in the DCEU might be, it's simply not a natural fit. Just as a future Joker 2 would risk tarnishing the strength of the original movie, including Joaquin Phoenix in the DC multiverse madness could undermine the legacy his Joker left behind.
