First off, I am a newbie to this game who quite literally just bought my first 750 points today. So just tell me if this idea has been done already.
Ok for the actually suggestion, basically you have a quest which requires at least three people. In side the dungeon there are three paths available to you. Each one of you would have to pull a lever closing the gate behind you but opening up the path ahead, note that you can only pull the lever once and the DM would have to tell you what the levers do. So basically as they progress, they will encounter objectives where, for example. their partners will have to say use a key inside a chest dropped by a red named monster to open a locked door with a lever inside. Pulling the lever would cause something which would make an invincible mini-boss vulnerable. Defeating this mini-boss would then push a stone wall which would reveal a mob and another lever, pulling this lever would unlock the door to the first party allowing them to advance. Then they would meet up for a boss fight and upon defeat of the boss the chest would appear.
In my opinion these types of quests would encourage people to group up and be self-sustainable.