Michael Keaton's Batman will be involved, as the Caped Crusader of the late '80s comes back into the fold.
The film will also see the return of Ben Affleck's incarnation of the Dark Knight as he reunites with Miller's Scarlet Speedster. While it is a fun DCEU reunion for the two actors, it's a tricky situation due to the 2017 version of Justice League by Joss Whedon. Hypothetically, The Flash movie does still follow the same timeline as that incarnation of the ensemble film. Unofficially, Whedon's Justice League has been de-canonized if recent comments from Wonder Woman director Patty Jenkins are any indicator. While Snyder's cut is officially getting its due in 2021 on HBO Max, the director has confirmed that his Justice League is officially not part of the DCEU anymore.
However, because of what The Flash movie is set to do, the Snyder Cut can still be recognized as being canon, but not on the same Earth. The Arrowverse 5-part event Crisis on Infinite Earths became key for Warner Bros.' DC properties going forward as they solidified their version of the Multiverse idea. When Miller's Flash crossed paths with Grant Gustin's iteration, the DCEU officially became part of the Arrowverse canon as being set on an alternative Earth. While Earth-DCEU didn't get an update after Oliver Queen rebooted the Multiverse, it definitely got restored since the film franchises are still going.
When Crisis on Infinite Earths aired, the Snyder Cut hadn't officially announced at that point, thus it was unknown that it would be outside of the DCEU. But now that Snyder has confirmed his Justice League is in a different continuity, The Flash movie can follow up on what the DCEU Multiverse where the Snyder Cut is somehow seen.
That way, it can establish why there are two different versions of Miller and Affleck's characters in live-action at the same time. That would especially be helpful for the Snyder-verse if it continues on HBO Max after Justice League comes out. Even if Snyder's continuity doesn't continue fully on the big-screen, the DCEU should still in one way or another find a way to properly acknowledge it. Since Whedon's controversial cut of Justice League that both bombed critically and financially should have never happened, the Snyder Cut should get its establishment through The Flash movie as the DCEU begins exploring the Multiverse at greater lengths.
