Nation doesn't dissapear just because it has lost its country. I think they'll rather just close to themselves and don't really interact with others, or they're banned ftom interaction with nations other than in country in which they live now, or maybe combination of both. Or they do interact with others, just not on political level. Take Slovakia as example - over 1100 years without own country and didn't dissapear at all, since nation was still there. Neither Baltics nor Russia's sisters didn't dissapear when they were part of Soviet Union, or

About fanfiction, I didn't hear about it at all, though once I read LietPol doujinshi about similar case.