Player Created Dungeons
TL;DR Super Mario Maker style end game dungeons.
Allow the player to craft a special item, like a descending staircase or waygate, that when interacted with loads the player into an instanced underground zone. Where they can expand by adding rooms, setting up spike traps or swinging axes, and more importantly, choosing spawn points for enemies like skeletons, wolves, bandits and bosses.
Each enemy would have a difficulty cost, while each floor and room would have a max threshold, but by placing a descending staircase another floor can be added, which would have higher limits.
The player could then share their dungeon with other players via either a code that can be copied or a website link, where dungeons could be rated.
Enemies wouldn't drop resources, but rewards could still exist with dungeon exp and levels. Increasing your dungeon level would allow the player to make more complex dungeons. With harder enemy varieties, more floors and extra backdrops being unlocked later on.