No way. Fairuza would be a terrible, terrible choice for Harley Quinn. The problem with her is that she already looks like a psycho. Hell, she IS a psycho. I've never seen her not play a psycho. She's scary... Harley Quinn might be a psycho, but she's not scary, at least not in the way she looks or sounds, or even behaves really. She usually seems much more like just a ditzy blonde, albeit one with a few major eccentricities. She never seems angry or evil, or sinister in any way. You wouldn't know or think she's a psycho to look at her. She's genuinely amusing, and completely non-threatening (until of course she actually starts killing people, or aiding her scary boyfriend in killing people.) Really, to me, the only thing even remotely creepy, or sinister about Harley Quinn is her twisted love for the Joker. If it weren't for that, I'd love to have her as the clown at my kids' birthday parties.
Fairuza Balk? She already looks, and sounds more evil than Joker, and certainly more threatening and sinister than Harley Quinn has ever been in the past.
And most of all, I still can picture her AT ALL as a psychiatrist. Maybe I'm not giving her enough credit, but I just don't think ANYTHING about her suggests that she would be a good Harley Quinn. You people seem to be basing the idea that she would be great at it solely on the fact that she's good at playing a raving lunatic. If that were a smart approach, then we'd have Crispin Glover as the Joker.