Is Warner Bros. upcoming Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) the start of a series of crossover movies between DC's premiere all female superhero team and their villainous counterparts the Gotham City Sirens? Arumor reported by The Hollywood Reporter seems to suggest that's the case, saying that Birds of Prey is the first in a trilogy of films that will culminate in a movie that will bring the Birds and Sirens together.
Couched in speculation about which villains may appear in director Matt Reeves' upcoming The Batman, THR cites a rumor saying Birds of Prey will kick off a three-movie series, followed immediately by the long-rumored Gotham City Sirens film starring Harley Quinn, Catwoman, and Poison Ivy, and capped off by a third movie which would pit the two teams against each other.
Gotham City Sirens was first reported as in development in December 2016 with Suicide Squad director David Ayer said to be on board to write and direct the film. Ayer later tweeted an image of the villain Black Mask, who was rumored to be the villain of Gotham City Sirens - however, actor Ewan McGregor has been cast as Black Mask as the primary villain of Birds of Prey, potentially suggesting plans changed entirely since that February 2017 tweet.
That was the last reported update on the project - until this new rumor. It's possible the original reported plans for Gotham City Sirens were adapted into Birds of Prey, leading to this reported expansion of the story into a series.
Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) is currently in production for a February 7, 2020 release date.