they should put harley quinn. in the movie she should start off being the replacement physcologist for dr. crane (the scarecrow) at arkham and police dept. then she meets the joker as a prisoner in arkham after being caught by batman. she doesnt think the joker is an insane lunatic killer, but rather a very misunderstood person, and the joker notices this and takes advantage of this. so he convinces her to secretly free him from arkham every time he gets caught, at first she's reluctant but then she easily gives in and starts falling in love. the joker is also aware that harvey dent is shcizophrenic and is also one of harley's private practice patients at the police dept and insists that she do reverse physcology on dent and inform him what happens on all the sessions. before the end of the movie, the joker asks harley on dent's whereabouts, she tells him, frees him from arkham one last time because an authority catches her, arrests her, she gets mad, and in order to escape she kills a few people. she then realizes why the joker likes killing so much and decides that she wants to be like him. she knows the joker is going after dent, and the cops and batman will be after joker. she then decides to find the joker in order to rescue him before he gets found and arrested..... then at the very last scene of the movie, harley finds the joker very beaten (by batman of course), hiding behind the building where the incident happened, in harleyquinn make-up and costume all done, and takes him safely away, thus the joker waking up and realizing that this woman is his lover, and the final shot being him making an evil grin and then laughing.
by the way reese witherspoon should play harley quinn.