How about, at the top of the mission screen, you have two or three 'Random missions' - Yes, with a better name. Basicly it just picks one out of a pool of special missions, in a manner similar to the NPCs on the battle tab. The missions would not appear on the standard list, but the rarer missions would give rewards you couldn't get any other way - the expire time could be about 3 days - 2 weeks, depending on missions, then, like elite NPCs, have some rare ones that you can ask your legion for help with, in the manner you would alert an NPC. All the legion players inside the rank range can spend energy/materials on the missions, and at the end, their is a reward that would befit the whole legion? Just thinking aloud.