Strategy based structure: the labyrinth.

This would be a structure that you need to prepare in advance for, similar to the deep dark's ancient cities. The structure itself would be as the name suggests, a huge maze, with lots of crossroads and loops back to other places, so it's easy to get lost. The walls would be made of a slow to mine block, making it quicker to just explore. Occasionally, you'll find a room with a chest with good loot. However, those rooms would be guarded... The Hoarder: a mob that generates in the loot rooms. They'll be comparable to the warden. They're big, strong, but slow, with bad eyesight. They try to stay close to the block they spawn on while idle, only moving away to attack the player. The best strategy is to lure them far enough into the labyrinth, then get back to the loot rooms before they do. It's also best to not leave the room blocked off after you leave it, otherwise the Hoarder will be wandering throughout the labyrinth, making it harder to navigate safely. Come prepared with a strategy to safely get the loot, and the unprepared will quickly learn their lesson.