In terms of drama building, I could see Zoey returning as either Chad's lover or Quinn's supplier.
As Chad's lover, this could create quite a bit of drama within the MC/Sage relationship, not only with the MC's feelings in terms of seeing his former lover/sort-of girlfriend with Chad, but also with Sage finding out she's the MC's former lover/sort-of girlfriend.
As Quinn's supplier, this makes her something of a villain in the story and could really rock the MC to his core to find out that she's gone down such a dark path, and make him conflicted if he has to go against his former lover/sort-of girlfriend in order to possibly help Quinn.
A possible third option is that something happens that really messes with his current life and he reaches out to her for advice, and either she shows up at the college or they chat over video call and she helps him through a tough spot as well as giving him closure on what they were to each other and could have been if she hadn't left.
But, as I've said, whilst I don't rule out her coming back at some point, I'm not holding my breath that it will happen.