What about a natural disaster? e.g. people fleeing after a volcanic eruption? How ASB could it go? Yellowstone forcing people to move south? Tsunami forcing people to move inland? Peshawar Lancers type disaster?
Well, manmade disasters (e.g. the Dust Bowl) are perfectly okay, but you can't have a natural disaster occur in a period of history where no such natural disaster occurred. Essentially if you're setting your map before 2011 then you can't change any historical natural disasters. If you're setting it after 2011 then the issues become fuzzier, as there are several volcanoes that
could erupt any day now, and it's rather hard to predict the location, date and severity of earthquakes, and so forth.
Suffice it to say that any natural disasters you want to have happen must be possible, so no earthquakes well away from tectonic plate boundaries, no asteroids that we should have seen coming randomly appearing and smashing into the Earth and so forth.
What about some off-Earth civilisation made up of people fleeing something that happened here?
That'd be fine. We don't know a whole lot about other planets, so there's a lot of room to fill in the blanks if you want to.
If geo-PoDs are out of the running, I would think that aliens and the like are probably way out. It does make sense, considering we just had an ASB comp.
I do not consider aliens to be inherently ASB - you can do them plausibly. If they look just like humans but with weird foreheads then that might be going too far, but generally I'm okay with the possibility of alien life.