The Flash's ability to time-travel played a major role in Justice League Snyder Cut. Following Zack Snyder's departure from the movie due to a family tragedy, the version of Justice League that was released in theaters was the product of major reshoots that took the movie far off-course from his original vision. At this point, the famed Snyder Cut of the DC superhero ensemble has risen to a massive level of public prominence, with demand for its release only growing with every social media tease Snyder makes for it.
The reason the Justice League Snyder Cut has received such a widespread campaign is because there's clearly so much from the original version of the film that was either tossed aside or whittled down enough for it become forgotten in the theatrical version. Much of this has to do with Superman and Darkseid, but a key example of this is also Flash time traveling. In the theatrical version, Flash doesn't use his time travel abilities - it's possible he doesn't even know he's able to travel back in time - but it was something that was previously teased in Batman v Superman: Dawn of Justice, and it would've played a role in Snyder's version of Justice League.
Snyder has recently offered a glimpse at Flash's time-travel abilities in the third act of the Justice League Snyder Cut. In Snyder's version of the film, Barry Allen travels back in time following the League's initial defeat to Steppenwolf to give them another chance to stop the Apokoliptian invasion. At that point, Cyborg was to enter the Mother Boxes to prevent them from forming The Unity, while being tempted to join forces with Apokolips with a vision of his fully restored human body and his deceased mother and father by his side (with his father, Silas Stone, having been killed earlier in Snyder's version.) Members of the cast and crew have also revealed further elements of movie's climactic battle, including Steppenwolf meeting a drastically different end.
Aside from the original ending of Justice League, there's also at least one time-travel component that was ultimately excised from Snyder's original plan. Specifically, the Knightmare future seen in Batman v Superman was to be revisited with Barry traveling back in time to stop Darkseid from murdering Lois Lane in the Batcave, which in turn would prevent Superman from succumbing to the Anti-Life Equation and siding with Apokolips. However, the polarized response to the dark tone of Batman v Superman lead to the Justice League script undergoing a rewrite before principal photography, with this element ultimately being removed, though the Knightmare reality would still factor into the story, with Cyborg experiencing a vision of it while interfacing with the Kryptonian scout ship during Superman's resurrection.
With the demand for the Snyder Cut's release entering the mainstream, Snyder's tease of The Flash time-traveling to undo Steppenwolf's victory is just one of many factors that has fed the growing push for Warner Bros. to give it the greenlight. At the moment, the studio has not made any public plans for the Snyder Cut to be released. Nevertheless, with the campaign for Snyder's version of Justice League growing louder practically every day, some type of public comment on it from Warner Bros. will very likely become a necessity sooner rather than later.
