I would like this as well. I have been thinking about this for a long time. I think it would be cool to have public events set at the tower in which the Guardians present will have the option of taking part int a "horde" mode of sorts. Guardians would be going about their affairs when suddenly the tower darkens, warning sirens and lights spring up, and the girl on the PA goes "Attention Guardians! The __________ (Vex, Cabal, Hive, Fallen) are at our walls!" And a "Join event" prompt would show. Guardians would then be teleported to the wall (or even a part of the actual city, since we never really get to explore the full city itself) and would have to fend off an invasion force of that enemy group. And it would be magnificent. It would be large, with up to 10 guardians (or more, possibly. Public Events can support that many) holding off waves of enemies (we are used to them coming at us in waves, afterall). Imagine the Battle of Helms Deep, only with Titans, Hunters and Warlocks fending off the hordes and defending the walls of the city. The event would last a long time, perhaps the same length as a standard Crucible match. Battle is over when either all the Guardians die or when the timer runs out. Guardians who make it to the end get a Rare or Legendary Engram. It could follow the same principle of the Outpost Attacks from the game "Dead Frontier." That's how I would do it anyway.