I disagree. I suspect the final biome will be the underground ruins of the society that in ancient times defied Odin's will. One of the rune inscriptions lays it out quite clearly:

"Beneath the ground are the halls of men and women long since gone, ancient tribes who lived in Valheim even before the Allfather turned his eye upon this place. Delve deep to find their treasures but beware, some amongst them do not yet know they are dead."

Conquering the remains of this ancient civilisation, the most stubborn and powerful of Odin's enemies on Valheim, would be a fitting climax to the game.