As for now there is nothing you can focus on once you defeated all bosses. The only thing you get are boring raids. But, I think, it would be nice to have random quests which will reward you with New Material needed to craft new weapons and armor or some other stuff, maybe some cool item from Trader, traded only with this material.
What kind of quests? For example: find and clear fulings village. Something evil respawned in one of the tombs, go there and kill it. Search for the super powerful troll and his minions roaming in these woods. Mini bosses in tombs, villages and crypst would be cool. Just give the monsters we know more hp and attack power, color them differently, and here we go.
Hugin and/or Trader could give us these quests, for example 3 per day. They would be completely random.
After clearing a specified number of them we would be given a different, harder task: to kill a boss, but this time much more powerful.
I think this is a pretty simple idea, but this would prolong Valheim's experience for many players. I would be more than glad if I had this kind of task to fulfill. My Valheim existence wouldn't be co senseless.