Honestly i really hope Summer is dead, or that if she is missing she will continue to do so.
I don't think the game really needs her return for more drama or story, the MC and the most LI's have allready more than enough shit to deal with.
Playing a mentally/emotionally wreck of an MC and him trying to get his life back, just for the source of all the troubles suddenly standing at his doors and saying something like 'hey, sorry i was in witsec, but now i'm back' would really piss me off to no end...
At least from my experience, the sudden return of a LI after a long time is rarely done well, most of the times the explanation for the missing time is implausible, has a lot of large plotholes and has more demerits to the overall story than it was worth bringing the LI back...last game i played which did that was LoF and at least for me it was really a weak solution with tons of plotholes which killed a lot of enthusiasm which i had for the game.
Amnesia can be interesting, but i think i only saw it done well a couple of times, all of them had either a central LI or a very small cast 2-3 LI's, so that there was a large focus on the LI and the whole amnesia theme.
In most cases is really poor done without much focus on the whole theme and relative quickly resolved...it basicialy screams 'well i had no other/plausible idea, so amnesia'.
There's also a third option that combines elements of both and I think would be the most interesting and poetic. Summer returns, but doesn't love MC anymore and doesn't want to be in a relationship with him. Perhaps taken to the extreme, she might find it weird how MC is obsessed with what she sees as a relatively meaningless childhood crush/romance. It could potentially range from Summer still wanting a platonic friendship to outright hositility or disgust. It would then become apparent that the Summer we see in the flashbacks is really just an idealised version of Summer in MC's memory of his idealised relationship. MC will have to deal with the shattering of his fiction and how he conceptualises relationships. Summer isn't dead, but MC's version of Summer is. Reminds me of this quote from the show Boston Legal:
Part of the reason I like this idea is because it basically subverts (in an interesting way!) the whole premise of the story, but also on a meta level, the wider genre of erotic/romantic VNs. The idea of a potential or assumed love interested just rejecting you is basically unheard of in this genre. I don't mean this in the NTR sense either, where we brutally denied (narratively) a relationship with Summer and we have to see her paraded around with other men, but rather that Summer returns, and doesn't play a significant role in MC's life anymore, maybe she leaves for good. Anyway just an interesting idea I had.
This is actually a interesting idea, which i never saw in a VN.
Probably one of the only settings where i would want to see a return of Summer.