Whether it's someone, or something, unexpected, I just hope it makes sense. I fear too many creators value a surprise twist over a logical revelation. It makes sense, a surprise is a lot of fun and keeps things interesting. But it can hurt the story if it goes against the established in-universe logic.
From where we sit right now, I think it's anyone's guess who she'll sleep with. In-game we're about a week away from when she'll cheat on Liam for the first time, and I don't think any contenders have really separated themselves from the pack. In my opinion, whoever it is needs to step forward pretty quickly or else it's going to feel rushed.