"#Justice League," a movie directed by Zack Snyder (with a little help from Joss Whedon), contains a number of Easter Eggs. From superhero catchphrases to references from comic books - the film was filled with details that only the most loyal of fans will be able to spot.

Some of these Easter Eggs subtly introduce the next films that are slated to come out from the DC Extended Universe in the next couple of years. One of the upcoming films that were set-up in DC's latest offering is "#Gotham City Sirens."

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To recall, last year, following the success of "Suicide Squad," Warner Bros. confirmed that "Gotham City Sirens" is being developed for the big screen.

The film will center on Harley Quinn played by Margot Robbie (who will also sit as an executive producer). David Ayer has been tapped to direct the spin-off.

Not much information is known about the upcoming film; Warner Bros. has not even announced the actresses who will play the role of Dr. Pamela Isley/Poison Ivy and Selina Kyle/Catwoman. However, Ayer has already teased the villain who will face the trio and "Justice League" has already set up his arrival to the #DCEU.

Comicbook.com noted that the beginning of the superhero team-up movie, "Justice League," featured a reference to Janus Cosmetics, a company ran by Roman Sionis. For the uninitiated, the character is a prominent villain of Batman. Comic book fans know him as Black Mask.

Janus Cosmetics made a brief cameo during the rooftop scene where Batman (Ben Affleck) pursued a cat burglar and faced off with a parademon.

Behind the action, a neon sign of Janus was visible.

Ayer has all but confirmed that Black Mask is going to show up in "Gotham City Sirens." Earlier this year, he posted a picture of the Black Mask on his Twitter account, making fans believe that the comic book villain will be heading to the big screen soon.

"Gotham City Sirens" is still in development

Meanwhile, despite rumors, "Gotham City Sirens" is still in development. Back in August, word on the web claimed that the film has been scrapped to make way for the big screen adaptation of "Mad Love," a story that centers on the crazy relationship between Harley Quinn and the Joker. However, an insider confirmed that Warner Bros. still wants to see the all-female anti-hero story come to life.

"Harley, Catwoman and Poison Ivy are super important to us," the source who spoke exclusively to Forbes revealed. "And a super cool team, too!"

After the critical and box office success of "Wonder Woman," DCEU will likely push for more female-centered superhero films. A Batgirl film that will be directed by Whedon is also in the works.