The Flash solo movie has been delayed numerous times, which has cast doubts upon whether or not Ezra Miller will still star in the DCEU film. Miller was first cast as Barry Allen way back in 2014, but five years later The Flash still hasn't entered production, after a very troubled development period.
Phil Lord and Christopher Miller wrote the initial story treatment, with Seth Grahame-Smith signing on to write and direct the movie in 2015 when they passed because of their busy schedule. Fast-forward to 2016, however, and Grahame-Smith departed the project due to creative differences. Rick Famuyiwa replaced him, with an eye on filming in early-2017, but he also dropped out just a few months later. Over a year of various reports and rumors followed - names linked ranged from Robert Zemeckis to Matthew Vaughn and back to Lord & Miller again - before, in early-2018, it was announced that Jonathan Goldstein and John Francis Daley (Game Night) had been brought in to direct. The latest official word is that The Flash will film after Fantastic Beasts 3, which releases in November 2021, meaning the film is still a long way off happening.
Through all of this turmoil, Ezra Miller has remained the one constant on the project. The actor has already played Barry Allen three times, with cameos in Batman v Superman: Dawn of Justice and Suicide Squad leading to a full appearance in Justice League. Miller's take on the character has proved popular, if not exactly irreplaceable. Miller seemingly remains committed to making it happen too. The Fantastic Beasts star reiterated earlier this year that it would still happen, and was even reported to be rewriting The Flash script too in order to keep the role. They aren't the actions of someone who wants out, but an actor who remains serious about making the film happen.
It's not as easy as the actor wanting it to happen though. The version of the script he was writing - alongside acclaimed comic book writer Grant Morrison - was said to be a darker take to the more lighthearted one planned by Goldstein & Daley. Since Warner Bros. have been moving the DCEU from the former towards the latter post-Zack Snyder, Miller's efforts might not be enough to keep the gig. Miller is a microcosm of that too, since it was Snyder who hired the actor to play The Flash.
There's also the fact that his contract reportedly expired in May (per the same story on the script rewrite), meaning that it'd need to be renegotiated. That's not impossible, but Miller is a much bigger name now than when he first signed on, which would place extra demands on an already prolonged development and a movie that, after so much time, could be considered a risk. If Miller and WB can't agree on either a creative direction or a contract, then it's hard to see how the actor will continue playing The Flash.
Working in Miller's favor, however, is the aforementioned Fantastic Beasts franchise. While he was a part of the first film, sequel The Crimes of Grindelwald ended with a huge twist that made Miller's Credence Barebone the absolute centerpiece of at least the next film, if not the entire saga. WB is still betting big on the Harry Potter spin-off saga, and that means they're going to need to keep Miller happy. If he wants to make The Flash in return, then there's a good chance the higher-ups will do everything possible to make it work. For now, though, The Flash solo movie remains at a stand-still.