If you’re unfamilar with Flashpoint then maybe you’re more familiar with The Butterfly Effect, in which a time traveller makes small changes to the world with big impact. That’s essentially the crux of Flashpoint and explains how we could see an (older) Keaton return as both a member of the Wayne family and Batman himself.
Moreover, The Hollywood Reporter says that Keaton could not only portray the character in The Flash but in a number of DC films, mirroring Samuel L. Jackon’s Nick Fury across Marvel films.
Keaton appeared as Bruce Wayne/Batman in 1989’s Batman and its sequel, 1992’s Batman Returns.
The Flash begins filming in 2021 and will star Ezra Miller as Barry Allen/The Flash, reprising his role in Justice League. On the Batman front, Robert Pattinson will play Bruce Wayne/Batman in Matt Reeve’s The Batman.