Samara Weaving I would use if more of a cute fun girl buddy take on the character.
Phoebe Tonkin's name has been floated out for numerous fan castings but none of them seemed to really be perfect I don't think until coming across this thread and Poison Ivy seems to fit.
Most of you might recognize her from "The Vampire Diaries" or "The Originals"
; she's friends with Margot Robbie and the two of them often go to fashion shows together so there could be good casting possibility there.
I couldn't find any photoshop jobs with her in the Poison Ivy role but do like this character poster of her that was made for season 2 of "The Originals"
(Details about this poster given at following link: aboutnicigiri: The Originals Season 2 Character Posters)
This fan rendition utilizing this poster I found interesting here as well:
Phoebe Tonkin Italia by KiraMartell on DeviantArt
The image evokes a kind of draconian stern look yet counterbalanced with a 2nd softer image in fan rendition linked above. I can see the actress putting some dimension behind the Poison Ivy character.
Margot Robbie might be keen on casting her and I think she'd gel well either in The Batman or a Gotham City Sirens/Birds of Prey sequel.