The Ezra Miller's Flash will be indeed heading into production but it's unlikely to begin filming until 2021. Miller made his debut as Bary Allen in Batman V Superman and he became a main player in the Justice League film.
Miller was supposed to do a Flash solo film with Rick Famuyiwa as Director but Warner Bros. and Famuyiwa parted ways because of creative differences. Warner Bros. tapped IT director Andy Muschietti to direct The Flash and Christina Hodson to write the script. Variety reported about the latest update of The Flash film.
"Warners and DC also still have faith in Ezra Miller’s smart-ass interpretation of the Flash and are proceeding with development on a standalone film based on the character... Production on “The Flash” won’t be able to start until Miller finishes up work on the next “Fantastic Beasts” film, which means that cameras are unlikely to roll until 2021."
The delay of the production of The Flash film is the commitment of Eza Miller to Warner Bros.' Fantastic Beast 3 which will begin production in Spring of 2020. Miller will be busy filming Fantastic Beast 3 next year. Ezra Miller flash is one of the best parts of Justice League film, so fans wants more of him on the film.