Much of the criticism surrounding "Suicide Squad" -- and there's been a lot of it -- has concerned the movie's treatment of Margot Robbie's Harley Quinn, a beloved but complicated character. As presented in the movie, the Joker and Harley Quinn's relationship can be described as problematic at best
-- in one scene, he's administering electroshock therapy to torture her, but by the end of the movie he's breaking her out of prison in what's presented as something to root for. Her shirt says "Daddy's Lil Monster," presenting their dynamic as one of paternal ownership, which is icky at best.
So what could give her help broaden her character in a sequel? How about a partner that's truly an equal. In "Batman: The Animated Series" -- the show that introduced Harley in the first place -- Harley Quinn strikes up a friendship with Poison Ivy, something that's continued into DC Comics continuity. (In recent depictions, their relationship is more than just a friendship.) Adding Poison Ivy (created by Robert Kanigher and Sheldon Moldoff) would give the "Suicide Squad" series another high-profile Batman villain, and, as a bonus, Ivy's already been a part of the Squad in the comic books, during writer John Ostrander's famed run.