When all the buidling has been done (a building of every type has been build, the claim area is full, and the fighting has worn off, the game quickly loses it's charm. Also when you have been robbed for the upteened time in judgement hour, but that is another thing altogether...

We should have somthing to do overarching completing the skill trees and the buildings. Sothing maybe in a political realm or with a career as merchant, but this would involve NPC's. MAybe somthing with a king, or missions, but something that will keep you wanting to play even though you are a high level player with a secure claim that has everything there is to have, fending off robbers without any problem at judgement hour.

I am not sure myself what it could be but I am fairly certain that it should have some AI / NPC's involved.

Maybe horse races, knights tournaments, man against bear fighting (on which you can gamble), big markets once every month where good can be purchases at better prices then with players ingame. I don't know, but something...