In The Flash's final act, Barry Allen realises that in order to restore his timeline, he must allow his mother to be murdered. It's unclear why he never thought to travel to the moment she died to see who did the deed, though that Reverse-Flash reveal was presumably being saved for a sequel!
Regardless, on his way home, the Scarlet Speedster makes one small tweak to the timeline which allows him to clear his father's name.
As he finds out in the movie's closing moments, that's once again altered his reality, leading to Ben Affleck's Batman being replaced by George Clooney's. Original plans had called for that to be Michael Keaton's Caped Crusader, making him (and Supergirl) a permanent fixture of the DCEU.
We've heard that it was DC Studios co-CEOs James Gunn and Peter Safran who came up with the idea of enlisting Clooney, but it turns out he wasn't their first choice.
As you can see in the video below, Kevin Smith has revealed that Warner Bros. spent months asking Christian Bale to reprise his role as Bruce Wayne from The Dark Knight Trilogy. The actor repeatedly refused (he's said multiple times he wouldn't play Batman again without Christopher Nolan) and the rest is history.
Bale showing up for that final exchange with Barry would have made a much bigger splash, especially as Batman Begins, The Dark Knight, and The Dark Knight Rises are a) fresher in the minds of fans and b) beloved, unlike Batman and Robin.
Based on the reception to The Flash, Bale made the right choice. After all, he was unlucky enough to play Gorr the God Butcher in Thor: Love and Thunder, a movie widely regarded as perhaps the God of Thunder's worst solo outing.
Check out Smith's comments in full below.