secrets and unique things to find.
but yes underground caves with example spiders, worms.. maybe the caves could be a kind of cave down ot the underworld via caves and in the bottom lava and a big underground world/biome.
But in general being able to dig a tunnel could be cool.
Maybe there could be a final biome that when you ascend into valhalla there is one final test before you can get into valhalla.
Example yes you did slay the bosses to get to valhalla. BUT maybe the gods have a final test for you before you can enter.
There should be some tests to complete such as puzzels, and maybe some test that the player can answer about the valheim world. ect.