The Flash still hasn't entered production, after a very troubled development period.
Phil Lord and Christopher Miller wrote the initial story treatment, with 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 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 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.
