If you were to ask any DC fan what story they would like to see adapted onto the big screen, I will bet that Flashpoint would be at the top of that list. For those of you who have never read Flashpoint I recommend that you read it and also watch the animated adaptation, but I will also fill you in.
Barry Allen wakes up one morning and discovers that he is in an alternate universe. In this universe, his mother is alive, he can't access the speed force, the Justice League is never formed, Bruce Wayne died and Aquaman's army of Atlantans and Wonder Woman's Amazons are at war with each other. The story is a great twist on our favourite DC heroes and villains, my favourites being Thomas Wayne as Batman and Martha Wayne as Joker.
They are hinting at it already!
I can honestly see this movie happening. It has been alluded to
TV series but it hasn't been explored. What if the DCEU are planning to incorporate Flashpoint for the Flash movie, and that's why the series has only touched upon it? We already know that the DCEU are creating a lot of restrictions for the DCTVU, what if this is another?
The inclusion of Cyborg into the DCEU as a founding member of the Justice League could be seen as another hint as Cyborg, in Flashpoint, is the world's quintessential hero. What would be the point in using Cyborg is he wasn't already established in a previous movie? Using him in Justice League would let audiences familiarise themselves with Cyborg, creating more of a shock when he appears in Flashpoint.
The main piece of evidence that alludes to Flashpoint is actually tied to a specific casting.
Jeffrey Dean Morgan is Thomas Wayne
In the 'Batman vs Superman' trailer it was revealed that Jeffrey Dean Morgan was in fact playing Thomas Wayne in a flashback sequence. Why would you cast a very popular actor in a minor role? Because you are going to use him in a future movie, which would be Flashpoint! Also, can you imagine how bad-ass Jeffrey Dean Morgan would look as a gun wielding Batman?