The duo are in final negotiations to pen and helm an untitled movie project centering on Batman villains Joker and Harley Quinn.

Jared Leto and Margot Robbie are due to reprise their roles as the villains, among the most popular characters in the DC pantheon and which they first portrayed in 2016’s Suicide Squad.

Insiders say that the plan is for this feature to go after the studio makes a sequel to Suicide Squad. The latter is on the development fast-track and out to directors. But the Joker and Harley Quinn movie is also moving fast as Warners has to contend with actors' holding agreements, say sources.

The film is described as a “criminal love story,” according to one source, featuring the two murdering maniacs who are cornerstones in the Batman mythos. One insider offered this: "an insane and twisted love story. When Harry Met Sally on benzedrine."

Additionally, the project falls within Warners' main line of movies based on their DC properties. That includes the current pics released and in production that feature Ben Affleck as Batman, Henry Cavill as Superman and Gal Gadot as Wonder Woman, among other characters who collectively form the Justice League.

Warner Bros. is simultaneously beginning to develop a line of DC-related films that will fall outside the established cinematic universe which sees actors and storylines threaded through multiple movies.

The new line, or possibly label, will allow for the making of movies that will stand apart and separate from those big-budget tentpoles.

And, in a move showing how popular the character is, a Joker solo movie is being developed as a potential directing vehicle for The Hangover filmmaker Todd Phillips. It is intended to star an actor other than Leto. (Requa and Ficarra's Joker/Quinn project is further along in development than Phillips' Joker project, at least at this stage, according to one source.)

This strategy will distinguish Warners’ superhero movies from those of competitors, namely Disney, which has a highly interconnected Marvel universe of films as well as Star Wars. The latter has the main saga, first begun by creator George Lucas decades ago, and is now making stand-alone outings that are, at least at this stage, intended to fill in backstory for the main and ongoing story.

Warner Bros. may not be expanding its universe as much as doubling down on the voracious appetite audiences seem to have for these characters at this point in time.

Ficarra and Requa previously wrote and directed I Love You Phillip Morris (2009) and followed that up with the Crazy, Stupid, Love, which starred Steve Carell, Ryan Goslin and Emma Stone. The duo worked with Robbie when they wrote and directed the 2015 heist movie Focus and the 2016 war comedy Whiskey Tango Foxtrot.

Ficarra and Requa also exec produce and direct episodes of This is Us, NBC’s hit tearjerker that is up for nine Emmy Awards next month.

The pair are repped by CAA and McKuin Frankel.

Aug. 23, 6:40 p.m. An earlier version of this story stated that Matt Reeves' The Batman was part of Warner Bros.' new label and would star a new actor as Batman, but it is unclear where that project will fall.