
Spider-Verse filmmakers Phil Lord and Christopher Miller were attached to DC's The Flash way back in 2015, developing their own plans for the film. Even though they wound up departing the project, they've still got their treatment in their back pocket in the hopes they'll get to make it someday.
"We had a very elaborate treatment," Miller says in an interview on the Happy Sad Confused podcast (around the 49 minute mark), with Lord elaborating that it was "pretty different" from the film that actually came to fruition.