Another really cool idea from another game, Ragnarok Online, included the creation of Godly Items..
Those were a set of quests that should be done by 100+ people, in sequence, and each individual account could only do it once.
At the end of everything, every Guild had a chance to create a super OP Item for themselves (which usually ended up with the guild leader though).
https://irowiki.org/wiki/God_Items_Quest
Although I despise the idea of such powerful items being basically unobtainable by everyone, I really like the idea of those server-wide quests where you have to coordinate to complete, plus it is something that is always there but, because few people know about it, it is hardly finished.