If Joaquin Phoenix is playing coy here and he is circling the Joker origin movie, perhaps the nature of what the project is is what appeals to him.
Joaquin Phoenix has flirted with superhero films in the past and he was even considered the frontrunner for some time to be Doctor Strange for Marvel before ultimately passing on the project.
But WB may be on to something with these standalone films that may not be part of or beholden to the greater DCEU shared universe.
This allows for one-offs that can bring in talent that doesn’t want to commit to a decades worth of films but can still bring unique visions and interpretations to comic book stories.