Justice League 2 and Justice League 3 would have sealed the fate of all the DCEU characters introduced after Justice League. Since its inception, the DCEU has aimed to build a broad cinematic universe populated with numerous DC characters while also following the main storyline of Superman, Batman, and the Justice League. Although the DCEU has had a bumpier ride than expected, it has managed to release various successful movies such as Aquaman, Wonder Woman, and the improved Snyder Cut.
Zack Snyder's original plan for Justice League, as well as his final cut of the movie, further developed the Knightmare timeline established in Batman v Superman: Dawn of Justice. In this apocalyptic future, Superman goes rogue under Darkseid's influence and Anti-Life Equation and helps him destroy the world. Only a handful of the world's greatest heroes survive, including Flash, Cyborg, Mera, and Batman. The only way to save the universe from Darkseid and the Apokoliptians is to send the Flash back in time, which confirms just how invincible Superman and Darkseid really are. This also has a bigger implication for all the possible DCEU characters future movies would introduce.
Movies like Shazam! and Black Adam were going to happen regardless of the success of Justice League. Since their title characters exist in the same world as the superhero team, their absence in the Knightmare future would have to be acknowledged at some point, or at least hinted at. Had the DCEU not met so many logistical issues, it would have been natural to keep expanding the shared narrative and introducing multiple heroes and villains. So powerful characters like Supergirl, Martian Manhunter, Mister Miracle, and the Green Lanterns would exist alongside Ben Affleck's Batman and Henry Cavill's Superman. However, the Knightmare future from the Justice League trilogy would remind the audience that none of them survived in the future.
Of course, the Flash would have likely changed the main timeline through time travel in a Justice League sequel, probably allowing all superheroes to participate in a final battle in the style of Avengers: Endgame. But still, the Knightmare future confirms that the most powerful DCEU characters could only survive in a timeline where Superman didn’t fall to the dark side. This is somewhat limiting to the whole DCEU and all its solo movies. It would have been more exciting to witness epic battles like Supergirl vs Brainiac, the Flash vs Reverse Flash, or the Green Lanterns vs Sinestro without knowing the result of their fights and the unfortunate fate of everyone involved.
Zack Snyder's Justice League hints at a somewhat hopeful future for the DCEU's Justice League. Whether a future continuation of their story is bound to remain in the mind of the fans or it does materialize one way or another remains to be seen. The good news is that the DCEU will keep developing new stories, and their outcome may not be as predictable anymore.
