Basic Idea "The Rescue"
The setup has to be heartbreaking and engaging. I wouldn't want to write too much or the story here because too much early detail spoils the final results(+Wall of text babies hate reading sad I know)

Phase 1. Massive push, setting up the kidnapping/presumed death of a "Loved Figure" Footholds need to be world wide epidemic pushing the player base hard to keep them put down.
1.2 The breakthrough and kidnapping/killing we failed

Phase 2.Push back, As the initial impetus of the planar offensive breaks, and the tide swings "rumors" of the victims well being surfaces, (Investigation quests, in all the zones to confirm)
1.2 Confirmed!! we haven't failed yet, Rescue prep begins Harvesting/Production quests to prepare our forces for the rescue,

Phase 3. The Rescue, Raids Rifts Footholds Dungeons players must suppress the enemy as a diversion to allow the rescue to happen.

There must be a chance that we can fail and consequences if we do.. and Huge Rewards if we succeed.. Maybe a Fail should = suppressed rewards in planarite/drops/gold and xp for awhile while Success = a Nice bonus.. Maybe titles should be given to those players that rack up a large number of participation points. But we need the chance to fail and the chance to win and it should be a challenge not just for the 50's but for all levels. It's not really a game unless there is a chance you can loose.