4) Huntress
Yet another character who has previously appeared on Arrow, the Huntress is actually a character more commonly tied with Batman. As the Harley Quinn movie will likely take place in Gotham, given the character’s ties to the city, it makes a lot of sense for the film to introduce her.
Helena Bertinelli was, unlike Batman, brought up on the wrong side of the law, as the daughter of a mob boss. She realized that her father’s criminal empire needed to be stopped, however, and idolized Batman for being the only thing they feared. Inspired by him, she became her own masked vigilante in his image.
Alternatively, DC could go with the other version of Huntress, who is the daughter of Bruce Wayne and Selina Kyle from Earth Two, but that’s probably a tad unlikely right now.