|Art by Bruce Pennington|
Genre titles are really imprecise things, so let me explain what I mean: A setting that looks like fantasy, but is in fact sort of post-technological science fiction. What would make it "hard" as opposed to the usual science fantasy is that it wouldn't resort to what are essentially fantasy concepts like extradimensional entities or psionic powers to do it. The fantastic would come from at least moderately more possible sources like near Clarketech ("any sufficiently advanced technology is indistinguishable from magic") nanotechnology, cybernetics, and bio-engineering.
I haven't really seen this out there in gaming. True, Numenera presents a world utterly drenched in nanotech that can be tapped like magic by the masses, ignorant of it's nature. But Numenera still has psychic powers and extradimensional monsters, though. What I'm envisioning is more like Karl Shroeder's Ventus (where the "spirits" animating the natural world are AI controlled nanotech) or the Arabian Nights-flavored Sirr of Hannu Rajaniemi's The Fractal Prince where spirits in ancient tombs are digital mind emulations and the jinn are made of "wildcode" malicious nanotech.
Beyond nanotech, monsters would be genetically engineered creations of the past or descendants thereof. Or perhaps genuine aliens. Gods would be post-human biologic or AI entities--or often some combination of both. Or figments of human imagination. Or leftover bombs.
Why a more "rigorous" science fiction masquerading as fantasy world than the usual Dying Earths or what not? No real reason other than it seems to me starting with far future science fiction and figuring out how it would be rationalized by a more primitive mindset might yield a fresher take on the standard fantasy tropes.