Some fic ideas I've had, starting with nations:
Do Norway, Sweden, and Finland still exist in Y90? There would still be Norwegian, Swedish, and Finnish speakers, but there would be no one left who really remembered what it was like to live in a unified nation of millions of inhabitants; most would know only their own fortified villages, or those close enough to reach by short sea voyages, though there would still be some older people (like Trond) who heard about nations from parents or grandparents. With no remote communication technology and all travel being dangerous, I think nations might well fall apart.
Sweden is the most unified of the three; 3/4 of the population lives in "Mora", which I assume means Mora plus various farming villages around it. If you add up the populations of the four largest cities, you get 18,550, leaving 2,350 Swedes who live in villages smaller than Lulea with a population of 350. I wonder how much attachment those villagers feel to Mora or the old nation of Sweden, given they had to run things for themselves for years or decades with no help or contact from outside.
In Finland, half the population lives on islands in the lake Saimaa. Finland has 7,700 of 10,500 in its four biggest cities, leaving 2,800 in villages smaller than 600. Again, how attached are those people to the capital city or the nation of Finland?
The most interesting to me is Norway. 8,150 out of 15,100 live in four cities. That leaves almost 7,000 in smaller towns and villages. Those could be a little larger than in Sweden and Finland, as they only have to be smaller than 1,100. But Norway has two large cities, Aurland at 3,600 and Leikanger at 2,200; Aurland is the capital, but is Leikanger happy about that? Even Dalsnes, at 1,250 but with its troll-hunter clan, might want to challenge Aurland for the lead.
Anyway, I can imagine stories about people in some of these smaller communities: maybe they're even now just being brought back into contact.
Maybe there are people trying to hold the nations together, and meeting either active or passive resistance.
Might there be judges "riding the circuit" to the small towns? (This used to be the case in America.) How much authority do people grant them, and why?
Has Christianity really disappeared everywhere, or are some of those little villages Christian communities or communities of other faiths?