DCEU's The Flash movie will explore the DC Multiverse on the big-screen — which is why Grant Gustin’s Arrowverse version of Barry Allen needs to have a presence. Earlier this year, Ezra Miller and Gustin’s respective versions of the Scarlet Speedster made history in Warner Bros.’ massive DC live-action franchise on the big and small screen. After years of making it seem like it would never happen, the Arrowverse and the DCEU finally connected as existing in the same Multiverse. In the fourth part of Crisis on Infinite Earths, Gustin and Miller had a scene that was the ultimate surprise for viewers everywhere. While the scene was brief, it broke a long-time barrier between the DC movie and DC TV universes that existed since 2014 when The Flash premiered on The CW followed by Miller’s DCEU casting.
Miller, who has played the fastest man alive a few times on the big screen, will soon be headlining his own movie that has been in the works for over 6 years. Despite not being a straight-up adaptation, The Flash will do its own version of the Flashpoint arc involving not just time-travel, but a bit of Multiverse shenanigans — that’s how Michael Keaton’s Batman ends up being part of the story. Miller’s Arrowverse cameo had a huge significance that may have possibly set up how his movie plays out. During his scene, DCEU’s Barry ended up making it clear that this was the first time he had met his own doppelgänger as well as traveled across the Multiverse.
Miller's Barry didn’t even have the name The Flash until hearing it from Gustin’s Barry — who basically gives the cinematic speedster his superhero name. The CW's version of the DC speedster thus introduces and confirms the Multiverse concept to his DCEU counterpart, who had apparently speculated on this being a possibility. While details on the feature film are being extremely kept under wrap, this will be the DCEU's biggest exploration with the Multiverse. While it's unclear what DCEU's Flash was up to when he met the other Barry, it seemed like it was before meeting Keaton's Batman and getting the new suit that he'll wear. If this is the supposed timeline of The Flash movie's plot, then that is a testament to why Gustin's incarnation needs to show up.
If the movie doesn't include or reference the scene in Crisis on Infinite Earths, it would be a major backstep for the DCEU. Now that it is in the Arrowverse's Multiverse, the event is expected to be canon, especially now that these Flashes have met one another. It's not that Gustin's Flash would need to have a huge part — there just needs to be a scene worked into the movie that includes him. Not only does this have to be seen as canon for the DCEU's Barry, but it also has to get a proper answer to how that version of Barry ended up in the Speed Force with a doppelgänger version of himself. The moment in Crisis on Infinite Earths put emphasis on three major accomplishments for Warner Bros. and their DC properties. Not only did they manage to connect everything past, present, and future for their live-action franchises, but they managed to connect TV and film in an organic way.
But ultimately, it became a milestone for both of their versions of The Flash and highlighting what Gustin and Miller can do with their characters across their entire DC multi-media projects. More importantly, though, it began a new character arc for Miller's character that now needs to be properly serviced in his film. Even if Gustin were to only show up through that same scene, that footage is crucial to The Flash movie more than ever. It'd certainly not hurt if they perhaps have a new scene later in the film after Barry fixes his Flashpoint. But that Arrowverse moment with Gustin's Barry Allen has, no matter what, to be in The Flash film to help DCEU's Flash truly begin his new chapter as the big-screen Scarlet Speedster.
