One of the things I think is lacking in most post-holocaust RPGs is the “fish out of water” element. That is, most such games, like Gamma World, have the player characters come having grown up in the setting, many with mutations, etc. I think such settings are made for a scenario which envisions people from the modern era somehow being thrust into the post-apocalyptic world, and having to explore it and deal with the changes the cataclysm has wrought.
- Time Travel. The player characters are transported into the future by a time machine. Unexpectedly (or perhaps as part of the plan), it is a one-way trip, and they are now stranded. [The Time Machine, sort of; he chooses to stay, rather than being stranded.]
- Time Dilation via Space Travel. The player characters are astronauts traveling at near the speed of light. Thanks to the time dilation effect (which is a real thing, btw), time travels much more slowly for them than it does on Earth. When they return, many more years of objective time have passed back home than have passed subjectively on their ship. [Planet of the Apes.]
- Cyronics (or other means of suspended animation, such as induced hibernation). The player characters are frozen and then revived (via some automated process, or perhaps by their discovery by natives of the future world, who revive them accidentally or purposefully). [Genesis II and Planet Earth.]
- Isolation. The player characters are the descendents of survivors of the holocaust who took shelter just before the bombs hit (or the plague happened, or whatever the apocalypse happens to be), and who may or may not have maintained their culture as it was at the moment the world ended. Perhaps there are some gaps in the knowledge that has been transmitted– perhaps the residents don’t even realize there is a world outside to explore until something happens to force them out of their sheltered environment. [City of Ember, Logan’s Run, and Wall-E.]
They key to the “fish out of water” element is that the player characters haven’t witnessed the holocaust first hand. They’re not merely survivors, but find themselves in a very different world from the one they knew. If I do a Gamma World game, I’d definitely want to structure it this way.