It may be able to be created as an event or side quest or maybe just put in some part of the world.
Here's a link for you to have an idea of what's it is about:
LINK
Search for Ancient Cave.
That ancient cave was a part of a side quest of the game, where you could get rare treasures and items from it. To make it more challenging here, a cooldown could be implemented so that you can enter into the cave once every week. The idea is to make is impossible to be accomplished by any build or party (personally, i think that it'd be funnier if we could only run this solo).
Also,it'd be nice if it could have the same or similar rules as the original Ancient Cave, you can see the rules in the link.
I just throw the idea into the thread for you to make your own idea of how it could be implemented into the server, i think it'd be nice, because at least i have never something like that in any game, except for something similar in FF7.
Off-Topic: A server based on the ancient cave with permanent death would be kickass. XD