Mmmh, maybe the whip or the outfit, these sound promising. Thanks for the suggestions! ^_^
Ooh, a collar. Reduces willpower to nothing. Can only be taken off by the person who puts it on, but only after a day. Erica could put it on herself but could take it off later. She could put it on Jin to reset his dominance, but only if she hasn't already gone too far and he puts it on her instead, after getting her to humiliatingly reveal her plans.
Or maybe a strapon, which if you put it on Lea enables her to take Jin's path instead! If Erica puts it on she gains willpower.
Simple, maybe ropes for the slave's/Erica's bed which just add a few more scenes.
Mannequin stand, to keep a slave/Erica on display, with a scene or two.