In case you forgot, there are currently two competing stand-alone Joker movies in the works.
One is from Crazy Stupid Love's Glenn Ficarra and John Requa, who are crafting a "criminal love story" focused around the relationship between Jared Leto's Joker and Margot Robbie's Harley Quinn from Suicide Squad. The second is a gritty "origin story" from Todd Phillips (Old School), that has been described as "super dark and real" by former MMA fighter-and-now-actor Brendan Schaub. It's debatable whether or not Schaub knows that the Joker is, in fact, a fictional character.
Supposedly, Warner Brothers is looking at the project as a prestige picture (writer's note: I just work here, man), and was at one point courting Leonardo DiCaprio to play the infamous Batman villain. That must not have worked out, as they're now in talks with Joaquin Phoenix to slather on the grease paint and menace Gotham City. Martin Scorsese being attached as a producer has got to be the name attracting all these high-profile actors, though a script co-written by Scott Silver (who has an Oscar nomination for The Fighter under his belt) might actually be pretty appealing, too (not to mention the hefty paycheck that will undoubtedly be involved).
The thing is: Phoenix is generally regarded as a serious thespian, what with starring in Paul Thomas Anderson movies (Inherent Vice, The Master) and currently headlining auteur experiments like Lynne Ramsay's You Were Never Really Here (which is teriffic, and basically gives Joaquin his Taxi Driver turn). In fairness, the Joker has obviously earned other great performers a trophy nod (Jack Nicholson snagged a Golden Globe nomination for Tim Burton's Batman), and even a posthumous Supporting Actor win (Heath Ledger for Chris Nolan's The Dark Knight). Essentially, this is a "stranger things have happened" scenario.
What's still most baffling about this project is the involvement of Phillips, who crafts collections of moving images that resemble movies, but are really just trash piles that you watch while contemplating the finality of all existence. Why anyone would hire him to even shoot their wedding, let alone a pulp prestige project, is beyond me. Fuck that dude's whole bro-glorifying filmography.