The idea is to have the "safety net" of civilization gone. Whether civilization got blown up by nukes, or civilization was a race of zombie squids and most of them died when the oceans boiled away, it's about surviving and thriving by yourself, or with a small group, after the event. As long as there's no sense of, "all we have to do is survive until help arrives".Things like this are making me less sure about the "open to interpretation" line in the first post, so I should probably ask for clarification.
So the traditional post-apocalyptic setup in this context would be "catastrophe befalls the denizens of the overworld, which you then have to rebuild". Does the crisis have to have happened to the overworld, specifically? I ask because I'm working on a pack where instead of "the forces of <hell/aliens/natural disasters> have screwed up the overworld", you have something like "the forces of the overworld have screwed up the other planes". The player is basically a minion of the Nether trying to restore his home to its former glory.
Would something like that be eligible? Thanks in advance.
So yeah, alien invasion, even one by "normal people" on your home of "nether people" is fine.