I have thought about it and it would fundamentally change this game. Some people have already mentioned some of what would be needed. New enemies? Does day and night cycle matter underground? it would be pretty lame if there was nothing underground, so that would mean a whole new biome (and all the work that would have to go through to make it). They would also have to re-design how water works in underground (currently it wouldn't be good.

It is a cool idea but it would not work in Valheim