Given how they have shown prachi being more strong of late and now made ranbir weak and helpless due to pallavi, I think it will be prachi, as there will be nothing stopping or making prachi helpless, especially if she starts hating ranbir if he reveals the deal he made with rhea. Then she will hate both rhea and ranbir and that can initiate prachi hating both and wanting revenge from them, but I don't know if we can expect such good writing and strong characterisation from anil nagpal.
No matter how strong she is,I don't think she is the kind of person to take revenge. But if her hate is so fuelled against Rhea and abhi then for her mother's sake she might take revenge and fight. But again,not very sure she'd do this because it's prachi. But I do hope she gets ready to marry someone else and Ranbir intervenes and has a showdown in front of everyone.