Ivy is not so difficult. A pale, beautiful redhead femme fatale who's a gifted and driven scientist with specialties in genetics, botany and mycology. She could use modified fungal spores of the Ophiocordyceps fungus to control her victims; a genetically modified organism like that capable of getting into the 'wilds' of Gotham (later the world) and replicating would be *extremely* dangerous.
If you wanted to get a little more surreal or campy, a side effect of her immunity to said organism could turn her skin green if she had to make herself into a chimeric plant/human hybrid. Could also introduce a weakness wherein she needed full spectrum light for photosynthesis, burned in direct sunlight etc. etc.
You read it here first. Waiting for my royalty cheques, Warner Bros.