Warning: major spoilers for Batman v Superman: Dawn of Justice are ahead!
On top of featuring the two eponymous heroes in conflict, and Wonder Woman entering the fray, Batman v Superman: Dawn of Justice also saw cameos from Aquaman, The Flash, and Cyborg, but of those three, Flash had a little bit more to do. After enduring his "Knightmare," Bruce Wayne saw a future version of Barry Allen deliver a cryptic warning in yet another dream. By itself, this moment was weird and confusing to those not familiar with the comics, but fortunately moviegoers will get more context about what it meant later on in the DC Extended Universe.
Rather than incorporate this scene into the script’s world building ahead of time, Breaking Bad-style, producer Deborah Snyder explained to Collider that Flash’s cameo in Batman v Superman: Dawn of Justice was added during production as a tease for future events. She said:
This was kind of the opposite [of Breaking Bad] because we had the script and it wasn’t in the script. As we started then we worked on our script, I think we were in the middle of shooting and we started working on the outline for the next movie and where they go, and Zack said, ‘Oh my God let’s add this moment that is gonna pay off down the road, and we’ll find out more information.’
Crackling with energy and wearing futuristic armor based on his classic costume, Flash told Bruce Wayne that Lois Lane was "the key," and when he realized he arrived "too soon," he said that Bruce was always "right about him" and to "find us" before Bruce finally woke up. Although it’s unclear whether the "him" was referring to Superman or someone else, the "them" was the other Justice Leaguers, which is why Bruce sets out to find Aquaman, Flash, and Cyborg following Superman’s death. Given that this moment directly impacts the entire DCEU and not just one hero, next year’s Justice League: Part One seems like the best bet for learning more about Flash’s warning and the dystopian future he was trying to prevent. However, with Justice League: Part Two coming in 2019, perhaps it will be a few more years until we finally get all the answers.
Aside from that dream, the only other moment Flash was featured in Batman v Superman: Dawn of Justice was via security footage from Lex Luthor’s files, where Diana Prince saw Barry Allen quickly foil a convenience store robbery. It’s hard to tell whether Barry had already taken the Flash guise at that point or if he was still getting used to his super speed, but by the time Batman recruits him and the world’s other known "metahumans," he’ll need to be ready to fight whatever being or force threatens Earth.
Ezra Miller’s Flash will return to the big screen for Justice League: Part One on November 17, 2017, and following that, he’ll lead his own movie on March 16, 2018.