The Flash will use his powers in a new way not previously seen on the big screen when Zack Snyder's cut of Justice League comes to HBO Max in 2021. During a panel for DC FanDome, Wonder Woman director Patty Jenkins asked Snyder whether there was something exciting for the character coming up in the Snyder Cut, and Snyder told her that the character would be doing something with his abilities that would "interact with time and space." Given the context of a day-long event which has also given fans a sense that the multiverse and time travel would be pretty important in The Flash solo movie, the immediate guess is that it ties into that.
When Barry Allen (Ezra Miller) first appeared as The Flash, in Batman v Superman: Dawn of Justice, he seemingly popped out of a wormhole to address Batman. He gave a cryptic warning to the Dark Knight, which may pay off in some way during Zack Snyder's Justice League.
"You're going to see in Flash, in this film, I don't think that you've ever seen before," Snyder teased. "Something that has to do with his abilities, because you know he's a quantum character. He interacts with time and space, so you might see him do something to do with that."
Certainly one possibility is moving through the Speed Force to travel through time and/or the multiverse. Fans already know that eventually The Flash makes his way into the multiverse and meets Grant Gustin's Flash in "Crisis on Infinite Earths."
Snyder had to leave Justice League, only months before release and amid frustrating conflicts with Warner Bros., when his daughter passed away. At the time, Warner said that Snyder had hand-picked Whedon to do brief reshoots and put a cap on his version of the film. In the intervening years, it has become clear that's not what happened. As of a few months ago, Snyder was still telling reporters he hadn't even seen the completed theatrical cut of Justice League.
The movie was a critical disappointment and a financial disaster -- a clear case of trying to please everyone by retaining some of Snyder's elements and mythology while changing the tone to a more humorous and Marvel Studios-inspired take. It backfired, pleasing almost nobody, and almost immediately, fans of Man of Steel and Batman v Superman: Dawn of Justice started lobbying to see a cut of the movie that more accurately reflected what Snyder had set out to take the screen.
#ReleaseTheSnyderCut became a social media phenomenon, and for years was a rallying point for the filmmaker's fans, who bombarded basically every Warner Bros.-related tweet with messages in support of Snyder.
Then, as HBO Max was preparing to launch, the studio finally relented, promising fans would see Snyder's vision in 2021 and committing an extra $30 million or so to completing visual effects, reshoots, and other odds and ends. The Snyder Cut will be available exclusively on HBO Max, with no word on any possible home video or select theatrical releases.