I've been thinking about some quick and simple setting conversions that wouldn't require any (or minimal) changes of rules.
Basically, all the GM needs to know is the setting and none of the rules would need to be changed (possibly a very small addition to rules, for contagions and the like).
Here's a few that I came up with today at work (slow day):
Y: The Last Man: Originally a comic book series, but now turned into a TV-series on Stars (Disney+). The premise is somewhat simple; every mammal with a Y chromosome dies in a mysterious disease that happens simultaneously across the globe. The world kinda goes to shit after that (not because "we can't run society without men", but rather because half the population died in an instant).
It's a post apocalyptic story with a twist.
I suppose a similar type of thing could be done with just men surviving. Either way the world is basically screwed and trying to survive is a priority.
Wouldn't require any rule changes at all. You'd just have to read into the background material (And come up wit ha reason as to why the event happened, because I don't think it's ever made 100% clear in the comics, and the show hasn't gotten there yet).
The Last of Us: Zombie apocalypse with a twist? It's a series of games (2 so far) set in a post apocalyptic world where 60 percent of humanity is wiped out by the genus of parasitic fungus Cordyceps. Basically, there are several stages and types of infection that would have to be statted out in the rules, but other than that no need for additional rules or changes.
Zombies: Walking dead, world war z, dawn of the dead, 28 hours later... take your pick. You'd have to come up with rules for zombies, but other than that, not much needs to change rules wize.
Enviromental catastrophe Pick one. Anyone. I know it's a controversial subject to some, but exploring the setting could be fun. Waterworld comes to mind, but that would require a LOT of rule writing for aquatic settings. But you could do any other natural disaster on a global scale.
Modern WWIII Basically the same premise, but set in modern times. Obviously the USSR wouldn't be the antagonist, but Russia is still a threat and China is rattling its sabers. It would require a lot of updated technology, but the rules would most likely not need much changing.
Anyone come up with anything else?
Let us hear your alternate settings.