A variant of this is, well, Fallout.

The apocalypse happened. Some small number of people hid in shelters for generations. The shelters are now opening.

The Earthdawn RPG was based off this; in fact, in Earthdawn it was cyclical.

You get a lot out of it. The PCs get a base (the small shelter) with limited resources they can build up or abandon. The shelter can have some arcane knowledge, but not unlimited, and much of it fragmented and not understood. The surface can be in many hostile environments. There can be other shelters, which they have been cut off from for ages; hostile? not? who knows. Beings who survived on the surface. Shelters that are still sealed. Shelters that opened before yours. Ruins and relics of the ancient past that your library may have information on.

...

A bonus to this is I don't need an evil-race (tm) trope. The disaster could have been a world war, an experiment gone wrong, or whatever. There will be prejudice from the past, and there will be new evils; but not a people assigned the "these are the bad guys, kill kill" taped to their forehead.