How about while we are in the tower, an alarm would start sounding with the announcer saying "City wall under attack! All available guardians respond." You could then bring your ghost up and have the option to join the fight. Perhaps the fallen are making a daring push on the city with one of their Kell leading the charge. Guardians who opted in would teleport to a base of the wall and have to fight off the fallen and kill the Kell. Or perhaps on a map like twilight gap just outside the city. When it's all done the guardians get the same rewards as a standard event and get teleport back to the Tower. This would be a rarity but would add some excitement to the tower.