The Book of the New Sun series by Gene Wolfe.
The setting is really unique and evocative, characters wandering through something akin to a medieval fantasy setting, but the “magic” is the technological remnants of a dying, out of reach space empire, which are incomprehensible to the low-tech denizens of the world.
I think Monte Cook’s Numenera is pretty explicitly inspired by The Book of the New Sun, but I think it’d be cool to see a more storygame kind of take on it. The books are a weird blend of classic fantasy adventure novel and the narrator meandering around about philosophy and the alien world and all that, and I think something like a PbtA game could direct the tone more in that stop and indulge in some melancholic philosophizing, or really focus on how weird and alien the world is direction.