She could come back for pure drama generation. If she turns up and she’s in a bit of a bind... broken up with the boyfriend or whatever and need a place to crash she could be a centralised target for the other girls jealousy.
Writing the LI’s being upset he’s dating other people would be a clusterfuck to account for all the variations on who you can date. If the Ex girlfriend comes back all the routes could focus on her as the “im jealous, can we be exclusive” target.
The games setting itself up for MC to be dating other people and his chosen LI and that isn’t sustainable. Bringing in a consistent “target” of jealously (even if you’re not forced to be sexual with her, just her presence as the Ex living with him would do) makes writing 100 times easier. Every route could focus on her and not have to account for other LI’s.
Just an idea. Without her any consequences for dating half the campus becomes hard to write.... if he even wanted consequences for that. This isn’t a theory just a way she could be introduced and serve a purpose.
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.