Just have Batman fuck her, it's why Selina went straight. Seriously, she becomes an increasingly more prominent love interest and suddenly she's suddenly more heroic, eventually even joins a Justice League.
Now the same thing is going to happen with Harley, she and Batman are going to have some hot bat-fetishist on clown action in Heroes in Crisis and by the end she'll be an actual Justice League member. What, you thought she was strangling Batman with Wonder Woman's lasso? That's just the kind of kinky stuff they get into.
So yeah just have Batman fuck Poison Ivy, the more graphic and explicit the better. For science.