Although this is just a rumor, if proven true, it could be considered a SPOILER for Birds of Prey, so proceed with caution.
Although plot details are being kept under wraps and nothing is official, it is reported that the villain in the DCEU's Birds of Prey will be Black Mask, a Gotham crime lord who is a huge fan of torture. We don't know for sure how the villain will factor in to Birds of Prey, but if a new rumor is correct, his actions will bring the team together and he will be the final boss in the third act of the film.
In the comics, Cassandra Cain was raised by assassins and trained to continue down that path. According to Revenge of the Fans, Black Mask kidnaps Cassandra Cain because of her skills, as he sees her as a perfect weapon that he can use to do his bidding. Black Mask also wants Cassandra to, in a Sith-like fashion, become his new favorite assassin after Huntress broke with him when she found out he killed her family.
Huntress is now hunting down Black Mask for what he did to her, and the abduction of Cassandra Cain unites her with crime-fighter Black Canary and Harley Quinn, who is trying to move on from Jared Leto's damaged Joker. Meanwhile, Renee Montoya is hunting down both Black Mask and Huntress because of the blood on their hands.
According to the rumor, although the abduction of the young Cassandra Cain will unite the team of heroes and anti-heroes into an uneasy alliance, saving her will actually happen before the third act. But once she's saved, Black Mask will send his men after the Birds of Prey and this will lead to the film's climax where the heroes face off with Black Mask, and Huntress has her shot at revenge.
So if this rumor bears out, Black Mask will be the villain so dastardly that heroes, villains, former villains and everything in between unite against him to form the Birds of Prey. This also sort of maintains the Helena Bertinelli Huntress origin where her family is killed by the mob and she becomes a brutal vigilante. In this instance, their deaths will be by Black Mask's "hands" and make for a familiar, but often compelling story of a student turning against the master once the wool has been removed from their eyes.
I like this story for Black Mask because it still allows for a character like Harley Quinn, who is traditionally a villain, seem justified in acting heroically. The abduction of a child to use as a weapon makes Black Mask the greater enemy.
This also feels like with Black Mask as the villain, the story will be very grounded and not give in to that common superhero film pitfall of a third act that gets too big and with too much CGI. In this way it isn't the world that hangs in the balance, the stakes are much more personal.
Both Ewan McGregor and Sharlto Copely's names have been floated as possible actors to play Black Mask, who, based on this rumor, will be an especially despicable villain. Whoever plays Black Mask will join Mary Elizabeth Winstead as Huntress, Jurnee Smollett-Bell as Black Canary, Rosie Perez as Renee Montoya and Margot Robbie as Harley Quinn.