I’d go for a postapocalyptic setting and use the FUDGE system (1995 release because it’s a free download).
Here’s the setting:
The year is 2055. 23 years ago earth fell. Within 14 days modern civilization was completely eradicated by aliens. Why? Nobody really knows. All you know that humanity went down the drains. All that is left are small groups of people, hiding in abandoned mines, forests, you name it. You are just trying to survive. The cities are death traps, filled with the alien invaders, commonly known as Xenos, and radiation. Nobody never ever came back from a trip into on of the major cities. You live in a small settlement deep in the Bitterroot Mountains. You live day by day as small troops head out to scavenge and hunt the wildlife. Sometimes they don’t come back either.