Jessica Chastain would be perfect for Poison Ivy though Deborah Ann Woll from True Blood and Daredevil would be a good choice... known but not crazy famous.
Sophie Turner might be a good alternative, she might be considered too young at 21 (Margot is only 26 though) and she's already in XMen.
Catwoman... eh... Mila would be great, I can see her getting along very well with Margot and can nail the comedy. Eva Green would kill it though she might be considered too mature/serious, I'm sure they'll have some comedic tones in the movie given Margot's Harley.