Re: The Dark Knight to have a female villain?
^ This is actually how I would see Harley fitting in. Someone like Lisbeth Salander from "The Girl With the Dragon Tattoo" books, goth and a psychiatrist who works at Arkham Aslyum who becomes corrupted by her fascination with the Joker. I think it would work.
Plus, seeing how the Joker killed off all of Gotham's mob bosses, knowing she's the Joker's girl could allow cronies left behind to rally to her side.
A Tiger doesn't loose sleep over the opinion of sheep.