Yeah, the unfortunate truth is that someone like Ivy would probably end up physically or sexually abusing someone like Harley, just as much as the joker did. I feel like the Joker liked Harley and got something out of their relationship, even if he was an evil abuser. I don't think Ivy would have gotten anything out of her. She's, frankly, too malevolent.

On a full side note, I think one character that's only done occasionally right is Killer Croc. I like the idea that, really, he's kind of a standard, low IQ, violent criminal. Just in a Crocodile like form. Like, if he wasn't a mutant, he'd just be another violent, street-urchin, thug on the streets of Gotham. He's like a lot of criminals: he's a pissed off, emotionally unstable, resentful, violent, mess who really contributes more to his own suffering than anyone else.