The Flash would be a more special effects heavy type of movie than we expect Deadpool 2 to be and that appears to be one of Miller’s passions.
The Flash would still give him the chance to make the comic book movie he’d probably like to make, while also being able to have some fun with the movie, as he did with Deadpool.
While Flash doesn’t generally use the level of language that Deadpool is known for, Barry Allen is still known for being as fast a talker as he is a runner.
It’s sort of a brilliant idea when you think about it.