The film is positioned to introduce the "multiverse" concept to moviegoers, where assorted versions of the same character exist simultaneously on parallel worlds and occasionally interact with each other. It's a core part of DC Comics lore and allows the movie studio to build out multiple franchises at the same time while providing filmmakers a flattering amount of flexibility in terms of connecting or disconnecting their work to other movies.
Keaton last played Bruce Wayne/Batman in 1992's Batman Returns but quit the franchise after director Tim Burton was pushed out for Joel Schumacher ahead of Batman Forever. The Flash is being directed by Andy Muschietti, best known for IT and IT: Chapter Two. Ezra Miller previously played Barry Allen/The Flash in Justice League and Suicide Squad.