Kristin seems like a good choice actually, I remember hearing that Anne Hathaway originally auditioned for the Dark Knight 2 thinking that she was going to play Harley, that would nice to see. Mia Sara did an excellent job portraying Harley's more deceptive and vengeful side in the short lived series Birds Of Prey.
Hard to say who could actually do Harley's character justice
What's the characters name from Orange is the New Black that's like, constantly talking about her boyfriend Christopher? Always driving the prison vans and stuff? She's basically Harley Quinn already, save less murderous