The untitled standalone movie will see Leto serving as an executive producer in addition to his starring role. Variety adds that their are no plot details available for the Joker movie yet, but the current plan is that the movie will represent a path for other characters featured in Suicide Squad to get their own franchises. This marks the latest extension of the Suicide Squad universe after Warner Bros. previously announced that Margot Robbie's Harley Quinn would be getting her own movie.
Last fall, Warner Bros. announced that a Joker origin story separate from the Suicide Squad depiction and the current DC cinematic universe was also in development. But it was also emphasized (somewhat confusingly) that that origin tale wouldn't mean the end of Leto's version either. In fact, what it would mean is that multiple actors would play multiple versions of the Joker without overlapping storylines, with Joaquin Phoenix reportedly in talks to play the origin version.
kind of complicated.
As for the movie starring Leto's Joker, it's not yet known when that will go into production. Currently both Leto and the studio are in search of a writer. Also unknown is whether or not that will begin filming before or after the Suicide Squad sequel, which is expected to go into production at some point next year.