Good for anyone reading manga and see animes that's right ... A guild where everyone can create quests or get quests ...
the player needs an item that drops from a monster, but does not have time to attempt to drop, he places that mission with a reward in gold or items and if any player thinks it's worth taking that mission and goes behind and drops those items and complete this quest.
I do not know how the AI of the game will work, but it can also the NPCs that put quests in this guild (even the developers themselves, who think of any quest)
very simple, may have other mechanics too, I find interesting the players themselves also increase the amount of quests and things they can do in the game.
also leave your ideas