Say that the Nolanverse didn't end, how would Poison Ivy have fit in the Nolan Batman movies? maybe somewhere between Batman Begins and TDK.
As others have said, she'd be an Eco Terrorist code-named Poison Ivy who uses bio-toxins created from various plants she's discovered and bred herself. She'd be a noted botonist as well, and more than likely a member of the League of Shadows. Probably the one who discovered that little flower they made the Scarecrow gas from.
One thing is certain. Bruce would end up telling her his identity at some point during the film.