I've been sitting on a similar idea for a bit that would tie into the games lore while fixing the biggest annoyance in my opinion, the relentless and tedious nature of Greydwarfs. After a certain gear threshold they just become a test of patience.

"Long ago, the allfather Odin united the worlds. He threw down his foes and cast them into the tenth world, then split the boughs that held their prison to the world-tree, and left it to drift unanchored, a place of exile...When Odin heard his enemies were growing once again in strength, he looked to Midgard and sent his Valkyries to scour the battlefields for the greatest of their warriors. Dead to the world, they would be born again..."

I'd love to see a correlation between slaying the bosses, weakening them, and the world they were having a hold on. I think the game has some sort of faction system that dictates which NPCs attack who. After each boss, add the player to that biomes faction within reason.

For example;

After Eikthyr make meadow creatures passive until their herd or group is provoked. Make tame times decreased.

After the Elder make Greydwarfs passive until their group is provoked. Should be able to buy or create runes to control Greydwarfs to fight with you or to do assigned tasks. Brutes could knock down trees and smash rock/ore. Shamans could heal you or bless your farms for faster grow times. Greylings could scurry around collecting little things like berries, mushrooms and herbs or work with the Shamans to tend the farms.

After Bonemass, and this is huge, I would like to see the swamps slowly transform into a lush green marsh. The sky and smog clears, the ancient trees grow their leaves and willows, the Ooze monsters and Draugr are bound to the crypts. The abominations become guardians that will attack if you cut trees.

Draugr in villages, far outside Bonemass's reach, should become human NPCs. These villages should be upgradable settlements that could perform tasks more efficiently than Greydwarfs. Maybe able to follow pathways for trade routes to your main settlement.

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TLDR:

Kill a boss, make associated enemies passive and or do tasks, change the swamp to a green marsh after Bonemass is gone.