I imagine a cool structure that spawns naturally when generating a new game such as a boss tower or arc shrine for the chalice but this structure is used as like a pokemon champion league structure where if you find it in the server, there are 4 NPC's which are the elite 4 and then a 5th "Champion".
These NPC's would have pokemon based on the various elite 4 members through out the game series but the level of the pokemon could be based on the players max level pokemon (like bosses).
I envision the elite 4 all have 5 pokemon that are level 120 with perfect IV's and EV's and the "champion" would have 6 pokemon that are level 140 with perfect IV's and EV's (assuming the player has a level 100 pokemon). This structure should be really rare so that on average on a server that is 20k blocks by 20k blocks there will only be 1 of these structures. Defeating the champion could provide some sort of "champion badge" that is pretty much worthless but just a neat achievement or objective to have in the game.
The details can be reworked so it's balanced and reasonable but this is to get the ball rolling.