Margo Robbie looks the part but I hate current, over-sexualized characterizations of the Harley Quinn character. Just...no.
As far as the Joker/Harley thing goes, there's a lot of good meat there to get one's teeth into (just look at Dini's Mad Love), if they look at the origins of the character. I mean, a psychiatrist studies the criminally insane, meets the worst of the worst and schemes to get rich from it, goes for an outing with the madman and falls into a psychosis on her own. On top of CessnaDriver's point, it's a terrible relationship and it shouldn't be something that's glamorized. That's plenty interesting and rife with drama; there's plenty of potential for something genuinely great. Unfortunately, the talent and the people in charge just aren't there to see it and make it.
What I'd love to see a series of shorts, fly-on-the-wall style, like Quinn had put hidden cameras in her office to record her Joker sessions, and through the arc of these shorts we see just how she turns over to side with the Joker. The way I imagine it is like the Clarice/Lecter segments from Silence of the Lambs.