Director Zack Snyder’s 2013 film Man of Steel not only rebooted the Superman movie franchise; it also served as the launch pad for a shared universe of DC Comics big-screen adaptations (known as the DC Extended Universe).
Since then, superhero movie fans have been playing a guessing game as to what the over-arching saga of the DCEU and its movies (including both the Justice League team-up adventures and solo character features alike) is going to be.
The same goes for The Flash solo movie, even now that Rick Famuyiwa is directing instead of Seth Grahame-Smith.
Rumors about the larger narrative that Snyder and his various collaborators that point towards the famous Justice League supervillain Darkseid making his debut in the DCEU down the line.