Not much is known about what the Flash movie will contain, but all signs point to it being an adaptation of the iconic Flashpoint comics. In this story Barry Allen uses his speedster abilities to travel back in time and save his mother from her death. But this fractures the timeline, and the story sees Flash travel through the multiverse attempting to make things right.
Ezra Miller’s tenure in the DCEU is currently streaming on HBO Max, which will also be the home of the Snyder Cut. You can use this link to sign up for the streaming service.