This is an interesting debate, and while I have a position taken, I would like to add brain food.
We can consider Summer as Chekhov's Gun or as a McGuffin. And both positions have their valid points, personally, seeing the amount of clichés, prototypical developments and anime axioms scattered here and there, I'm inclined to think Summer is a Chekhov`s gun.
But, at the same time, I agree that it would be very valuable to the story if she were only a McGuffin.
My position is less of a Macguffin and more of a Keyser Söze, something essential but never what it seems or even revealed to be true. Same sort of device as whatever is in the briefcase in Pulp Fiction (although that is more of an elevated Macguffin). She's certainly more than the prophecy in Oedipus Rex.
My reasoning why I don't think she'll make a reappearance:
- as mentioned, it weakens the main development of Nika as a character. Rather than owning his trauma and learning to overcome himself/be at peace with himself, all his validation becomes externalised into the presence of Summer. Sure there may be more drama to her reappearance, but it's a very unsatisfying resolution to his underlying issues, and less character development.
- The tension of her not being resolved is something every single player (well those who care about the story anyway) wants to solve. Keeping the suspense and mystery strengthens the turmoil, and evokes our inner PI. This is also a cheap device, but more effective than a cat fight between Bella, Summer, Nami etc. with Nika as the prize.
-The ensuing cat fight (even if just simmering) would make this just like hundreds of other college AVN's. No thanks. I hope for a game where the main character isn't immediately fought over, and doesn't automatically get a free pass with every LI. It's already too much like that. Mila at least refuses to go out with him if he's already going out with someone. There will be enough chances (too many) for cat fights without Summer reappearing to stake her claim, and there is no need to invoke Summer to instigate them. So it would be a waste of her character. It then devolves into just another dating sim that started with promise.
- Ocean had a third game planned with a female protagonist. Maybe the story from Summer's point of view? I'm thinking of branching narratives in a David Mitchell/Ghostwritten kinda way, where one story leads to the next, but it jumps across rather than being a direct continuation. Complete speculation from my part, but it'd be pretty effective. ie SG has the same characters from WiaB, but they're no longer center stage - their dramas occur in the background now. Even though they'll have a major effect, we'll see the story from Nika's perspective of what is important, not from Leia's or Katie's etc.
In the end, it's Ocean's story to tell, and if he's planned it I'll just hope he avoids the obvious tropes.