I've always wanted Harley Quinn to be in the third movie. Joker doesn't have to be recast, they could simply show her taking a glimpse into Joker's padded room and her either becoming smitten or madly insane. But I don't know what Harley Quinn could add to the third movie other than what the Joker brought to The Dark Knight.
Ra's al Ghul should make a return.
Ra's al Ghul should make a return.