Whether it was a movie or a TV series do you think a large audience would respond well to the Joker and Harley relationship dynamic? I think if it was done very carefully it could be pulled off for one thing it would have to be believable and that would depend mostly on the Joker. For one he'd either have to be portrayed as a certain way from the start even before Harley shows up. I think he could either be very heartless and manipulative from the start (which is common for abusers) or he could be shown as very sympathetic and complexed (not an excuse but would explain a lot) and the Harley character would be just as important like should she show some symptoms of insanity before her breakdown or should it be like journey for her?
I personally think Jessica Alba could play the character pretty well her or Scarlet Johansson.