Been doing some research into Scandanavian folklore and came across some creatures/concepts that could be interesting additions. These are the Fossegrim, Draugen, and Vardøger.

Fossegrim

Wikipedia describes Fossegrim as:

…an exceptionally talented fiddler: the sounds of forest, wind and water play over his fiddle strings. Fossegrims can be induced to teach the skill… Fossegrim is said to be willing to teach away his skills in exchange for a food offering made on a Thursday evening and in secrecy: a white he-goat thrown with head turned away into a waterfall that flows northwards, or smoked mutton (fenalår) stolen from the neighbour's storage four Thursdays in a row.

This would be a neat non-hostile addition to Black Forests, especially if waterfalls were added along with them. It could be a rare encounter, not unlike the merchant. If you happen to be in possession of Lox meat (or perhaps a Lox trophy), or 4 cooked meat taken from another player's chest, you could trade for a fiddle. The fiddle could simply be a fun musical tool, a la Sea of Thieves, or perhaps it could have certain magical properties like attracting specific animals.

Draugen / Draug

Wikipedia describes Draugen as:

…the spirits of mariners drowned at sea. The creature is said to possess a distinctly human form, with the exception that its head is composed entirely of seaweed. In other tellings, the draug is described as being a headless fisherman, dressed in oilskin and sailing in half a boat…

These would be encounters on shorelines of Swamps, Black Forests, Plains, and out on the Ocean during storms and foggy nights. Instead of moving toward the player, Draugen could use the game's harpoon mechanics to prevent the player from running away or forcing them to find a way to break the line. The idea here would be to learn to stay away, or be prepared to kill it. On the ocean, it would be interesting to see Draugen slowly navigating busted up Karves that harpoon your ship and attempt to ram it. Make em scream, too, when you get too close. Why not.

Killing these guys could net you some cursed seaweed that could be applied to a Karve or Longboat to inspire some faster paddling speed. Or perhaps the seaweed could be crafted into a wearable item that boosts paddling speed.

Vardøger

Wikipedia describes Vardøger as:

…a spirit with the subject's footsteps, voice, scent, or appearance and overall demeanor precedes them in a location or activity, resulting in witnesses believing they've seen or heard the actual person before the person physically arrives.

This would be a sort of miniboss. Very rare encounter that can only proc in locations/biomes you haven't been yet. These would snapshot your equipment at the time of the encounter and spawn a spirit copy of you that will pursue you until defeated. Your classic Dark Link encounter.

Killing it can drop an polished metal mirror shard. Gathering enough lets you create a magical mirror item that, when used, starts a casting timer that will spawn a spirit copy of you that acts on its own. Mimicking its movements could lend certain bonuses. Like bonus damage if you attack after it attacks. Or, maybe they could be autonomous sentries with limited range for your base (probably a lot easier to make).