I feel like knights in a post-apocalyptic setting is an under utilized setting in rpgs. The sort of thing where in the mid- to far future, human civilization has gone back to something more like the Middle Ages. Often magic will have returned or old science will seem like magic. There is some pretty good source material out there, but the only game I think can think of is Mutants in Avalon.
Fiction-wise, we've got: Moorcock's History of the Runestaff, Christopher's Sword of Spirits trilogy, and Harrison's The Pastel City (less so the sequels), at least. Comics-wise there isn't an exact fit (beyond the adaptations of Moorcock's work), but Camelot 3000 is close. There is even an 80s cartoon and toyline in the form of Visionaries.
Stephen King's Dark Tower series does a bit of this, but also leans on Western aesthetic and tropes and does that a bit more. Into the Badlands likewise has an Western element, but also wuxia.