Mind if I throw one in?
if it's bouncing off of one of mine go ahead.
Not really, but the first reminded me of an idea I had for a post-apocalyptic game in which huge asteroids had struck the earth in the middle of World War I. Europe reverted to a feudal system and there were things like knights walking the earth with Brodie helmets and Lee-Enfields, trying to survive the horrors of feudal trench warfare and the Lovecraftian things that were living in the asteroids. Mechanical knowledge from the industrial era was revered, and machines that still worked were cherished as holy relics. I figured the main action of the game could take place on the British mainland. The idea was silly as fuck, but also grim in a way that only this image can properly convey: http://michaelkutsche.deviantart.com/art/The-Trench-94147716
I figured I'd run it in a heavily modified variant of D20 Modern, or maybe Pathfinder, but I couldn't figure out a way to balance it properly or handle pre-decimalized currency, so I scrapped it.
Pretty similar in some of the flavor. Just use PF for most things and sub in d20 Modern rules for ancient tech. That's pretty much what I would I do if was planing on running PF. And screw trying to be balanced from the beginning. If things get unbalanced adjust fire and carry on.
Like British pounds?
Yeah, that's exactly what I'm talking about. Since all the currency in Pathfinder is decimalized, I couldn't just pull a simple substitution. However, I could scavenge the purchasing rules from d20 Modern and replace owned currency with a Wealth skill. I think I could just use Appraise for that; it never gets used anyway and if Paizo could get away with unifying all the acrobatics skills, I can probably get away with unifying the financial ones. The only challenge here will be sitting down and figuring out the purchase DCs for all the items.