One is the Republic of Darokin, from the old Mystara/Known World setting for D&D. What I like about it is that it is a plutocratic republic, which seems ideal for an old-school game where the idea that money is everything is already built into the rules via the XP system. In other words, unlike a feudal system where (at least in principle) being rich isn't what makes you prominent, in a plutocratic society gaining wealth is what makes you a leading citizen. So there is a connection between gaining treasure for XP and in gaining treasure for socio-political power in the game world. This one has the advantage of being part of a larger world already made, so having context for things would be no issue. Additionally, it's designed for early edition play and I think pretty much any old-school rules could be used with it without modification.
The other is Clark Ashton Smith's Averoigne. I've just started reading these stories, and I love the setting. A dark, forested, haunted (fictitious) duchy in medieval France. In some ways this appeals to me since what little I've read seems to combine the "sorcery-is-inherently-evil" motif of a Robert E. Howard world, with (a fictionalized version of) a geographical locale that I'm familiar with and very fond of. Again, that gives this setting the advantage of being located in a larger, ready-made context (historical France). This one would require a bit more work, since there would be no PC races other than human. Though I could easily see this working with a sufficient number of classes like: Friar, Undead Slayer, Healer, Inquisitor, Knight-Errant, Archer, Soldier, Woodsman, Wanderer, Bard, Explorer, and Bandit. The extra work wouldn't be too hard, since pretty much all of these class types have already been designed by someone else in some form or another, and at most I'd just need to tweak them a bit so they'd fit Swords and Wizardry.
This is all just casual musing of course, and actually doing anything with either setting is a long way off. Still, something to mull over on a rainy Sunday afternoon.