The arc I'm fully expecting to happen at some point, though of course it is baseless speculation on my part, is Arare finding out about Itsumi and Rin and deciding to break them up by dosing Rin and making her engage in acts with someone else (given the Toudou sisters luck, might backfire on Arare herself). If that did end up happening, I could see two possible outcomes:
A: Rin is tremendously broken up about it because of how it goes against her self-image and tearfully admits it to Itsumi, who... kinda just goes 'Yeah, that sort of thing happens' and admits some of her own history in that vein which might or might not open Rin up to the idea of poly since she's apparently also like that.
B: Rin holds herself together and calls Itsumi up to say 'Tell me where you are, so I can find you and we can ____' Which the latter probably goes along with since she likes attention and loves when Rin becomes aggressive.