With the ups and downs of the DC Extended Universe, some fans are ready to start over. Inconsistent quality, increasing talk of non-canon DC films, and the impending departure of Ben Affleck have fueled the desire for at least a soft reboot to DCEU continuity, and the upcoming Flashpoint adaptation could easily do this. The comic event used time travel to change the DC comic universe, so the movie could easily adjust the canon of the DC movie universe.
If you haven't read Flashpoint, here's the short version: The Flash travels through time to save his mother from Zoom, his arch-nemesis. This causes sweeping changes to history, and even prevents the accident that gave Barry his powers. The Flashpoint universe also features Thomas Wayne as a gun-toting Batman and Martha Wayne as the Joker, since the Flash travelling through time apparently caused Bruce Wayne to die in the alley instead of his parents.
Related: What is Flashpoint?
The story was loosely adapted on the CW's The Flash at the beginning of season 3. Fans weren't too happy with that version, though, since the entire story was condensed into a single episode - though it did allow for a few retcons and changes in the Arrowverse. But while using the movie adaptation to reboot the DCEU might seem like an obvious solution, it could actually make its problems even worse.
This Page: Why People Think Flashpoint will Reboot the DCEU
Why People Think Flashpoint will Reboot the DCEU
In the comics, the "Flashpoint" story arc was the immediate precursor to the New 52. This was the event that saw a reimagining of almost the entire DC Comics universe, with slightly younger heroes taking on new challenges and seeming to have no recollection of the decades of stories that came before. At the time, it appeared to be a massive reboot of the company's entire continuity.
Even in the adaptation that occurred on The Flash, "Flashpoint" had some lasting effects on the Arrowverse. Characters who were previously dead came back, and other slight changes occurred. The impact wasn't as severe as the creation of the New 52, but it was worked into the various shows nonetheless.
The storyline's history as a reset button of sorts is why many fans believe that the DCEU will use it the same way. It would give the studio an easy way to recast Ben Affleck (and potentially other characters) without the abrupt change that was seen when the MCU replaced Terrence Howard with Don Cheadle. Bits of continuity from the previous DCEU movies could be changed or ignored at will, and it would free up future movies from the backstory built into the previous films, marking those items as things that were changed by Flashpoint.
In theory, Warner Bros. could fix all of the problems in the current DCEU by simply having Flash rewrite the canon. The other films would still be considered part of the DCEU, but new films wouldn't be bound to them because the entire universe's continuity could shift at the end of Flashpoint. It would create a natural shifting point for a "Phase 2" of the DCEU, letting the new phase grow into the "new" setting for the films. The change could even set up the non-canon DC movies, allowing glimpses into alternate realities within the multiverse.
But Warner Bros. definitely shouldn't take the easy way out.