Don't know if she would be good or not, or if Harley should even be included.
Maybe she would be like Ledger with the Joker.
Anyway I found this:
an interview from http://www.**************.com/
QUESTION: If you could play one comic book character on screen, who
would it be and why?
ROSARIO DAWSON: Harley Quinn. Now THAT would be fun. I’d have to
really transform myself, which could be really, really fun. You don’t
see too many female villains that aren’t like Poison Ivy, where
they’re just really sexy but not much otherwise. Harley is such an
off-base kind of girl, she is in that same sort of Batman/Joker
insanity.
I love playing women who are strong, and characters that have these
great convictions and do the right thing. I’m always 100 percent
behind that. But I also like to challenge myself to kind of go in the
other direction and really explore what the opposite sort of
incarnation would be, to look at a situation and think, “What’s the
way I can reap the most harm.” You hit the most conflict when you’re
approaching a situation trying to be good, and it would be really
interesting and I think really freeing and liberating in some ways.
The physicality of doing it would be an interesting exploration
because you really can’t be too big or too full or too expressive when
you’re creating mayhem and embracing chaos. There’s a flow that evil
characters seem to get into sometimes that would be fun to play with,
like when you’re playing a
video game and you choose to be the
character that’s most destructive. I wouldn’t want to be that in real
life, I don’t want that legacy, I don’t want that reality, but it
would be interesting to explore it in a manner like that, to see what
it would feel like when that inner voice shuts off and you just kind
of go with it. That would be a safe environment to explore the darker
sides of myself.