According to storyboard artist Jay Oliva in August 2023, in addition to Snyder's intended "Darkseid quadrilogy" that would have ended up with what he described as "a Justice League Unlimited version" of the "Snyder-Verse", Warner Bros. planned to make a Flash trilogy, which was at different points of development, with directors like Seth Grahame-Smith and Rick Famuyiwa being considered, that would have culminated in a "Flashpoint" film adaptation similar to Oliva's D.C. Universe Animated Original Movies (D.C.U.A.O.M.) film Justice League: The Flashpoint Paradox (2013), which would have revealed that Professor Zoom had been behind the D.C.E.U.'s events, having been in the background during most of the trilogy while having some influence over the Justice League films' events, provoking the Flash into changing the universe in a way that resulted in everyone becoming enemies with those who used to be their allies, forcing the Flash to reboot the franchise in a way that could have introduced a new cast, due to the studio folks feeling that the starring actors would wish to move on from their roles after a decade.
Well, with The Flash having been inspired by the "Flashpoint" story-arc and, albeit in a totally different fashion, totally revamping the D.C.E.U. into the D.C.U., coupled with how they mucked the franchise up and gave Snyder the finger, unfortunately, it looks like Warner got their wish, just much sooner than initially anticipated.
Old news aside, here's my source: https://www.inverse.com/entertainment/zack-snyder-original-flash-movie-plan-jay-oliva
Shocking, isn't it?