Maybe they lead to a world boss, they reset every week guardians have 6 days to complete the quest line to get a chance on the 7th day to fight the world boss who protects a treasure all guardians who have completed the bounties get a gift in the post master only if the boss is taken down, the boss is only accessible via an extra mission selection after the bounties have been completed Have the boss as massive health massive damage, and have its in a stand alone instance so it should take the combined effort of all the guardians to take down, even if you leave the mission and return later it's still at the same health or less if other guardians have been hammering at it