The DCEU's Justice League founding in Batman v Superman: Dawn of Justice. Apart from some security footage of Barry Allen's escapades, The Flash also paid a quick time-travel visit to Bruce Wayne, during which he issued a warning that has yet to be fully explained.
Several years after his DCEU introduction, Ezra Miller's Barry Allen is still relatively a novice superhero. He only had a short stint as a vigilante before ing the Justice League, and whereas Flash had a long superhero career before starring in the multiverse-wrecking Flashpoint storyline in the comics, the DCEU's Flash will take a big jump and explore the DCEU multiverse, travel through time, and possibly make definitive changes for the whole franchise in his first solo movie. The Flash will also meet both Michael Keaton and Ben Affleck's Batmen, which is the perfect opportunity to answer a burning Batman v Superman question.
The future Flash that appeared before Bruce Wayne in Batman v Superman: Dawn of Justice warned Batman that "Lois is the key," before he realized that he had arrived too early. This scene was intended to be a tease of the Knightmare future, where Lois Lane remains the key to tame a power-corrupted Superman, who's brainwashed by Darkseid's Anti-Life Equation. Now that the DCEU is moving away from Justice League's original sequel plans, this set-up seems to have been abandoned. Even if the DCEU were to ignore Justice League moving forward, that scene between Ben Affleck's Batman and Ezra Miller’s Flash still exists within canon, waiting to be addressed by a movie like Andy Muschietti's The Flash.
How The DCEU Can Pay Off The Flash’s Knightmare Warning In BvS
The reunion between Ezra Miller's Barry Allen and Ben Affleck's Batman in The Flash, coupled with the movie's use of time travel and the multiverse, can clarify the meaning behind Flash's ominous warning in Batman v Superman: Dawn of Justice. Since the Knightmare future is unlikely to be revisited in entire DCEU universes can be erased with the right (or wrong) decisions, and that Darkseid's future reign has been averted due to Superman's sacrifice or Batman's change of heart after Batman v Superman: Dawn of Justice.
Given that The Flash will feature at least two multiversal Barry Allen variants, the movie could also provide a brief look at Knightmare Flash in order to confirm what fate awaited him, and whether his visit to Dawn of Justice's Batman was the right move. The Flash is the perfect place to tie loose ends. If not aided by a Henry Cavill cameo, The Flash could at least provide an update about Superman's off-screen status in a timeline where Flash's warning wasn't necessary. This could be the last time the DCEU could acknowledge Knightmare Flash, as a shared appearance between Ben Affleck and Ezra Miller is unlikely to happen again anytime soon after their appearance in The Flash.
