I know we saw Egypt when Praimfaya(not sure if that's right) was first shown. It will feel really open world if there are other settlements in other places on earth and they also had their own struggles and issues and ways to escape the fallout. Clark's Eden is the only inhabitable place we've seen so far on earth (this is unlikely, it's far more plausible that there are other pockets of land too). Now the fact that for now, it is the only place that can support life could be a way to bring in other people on earth. Maybe not now, but after the prisoners story is done, they could open the world up.
I would have thought this could be something they would do with this show, introducing nations and such. But, the grounders are descended from the second dawn bunker, aren't they, so unless other nations had bunkers that people went into, I doubt that they would be able to have survived.