Dungeons that only open IF / when a community solves a certain hard riddle, by community effort!
This riddles could be designed by devs, every certain amount of time period (not too often), and then when riddle is solved, dungeon is unlocked for a period of time.
After this period expires dungeon will lock itself, and after certain amount time has passsed, devs could design another riddle, so community can try to unlock this dungeon again.
What is a riddle? Well it could be a combination of tasks that need to be made, some of which could be announced, and some secret ones that people should first discover through exploration. Perhaps even some "brain stuff" through coordination with special NPCs, which would give you tasks and reward with some items in exchange that you could then need to find the use for...
Much could be done here. And this could be event like, not happening too often, and a dynamic, since devs will always place in initial riddle, so internet will not help you. You will always need community effort to unlock a "dungeon" again.
Dungeon - labyrinth, asteroid,...