I think it would be cool if Hive tried to invade and everyone inside the tower gets there point of view changed to 1st Person and has too defend the tower. They can have this as a random event, would be fun to have at the tower. Bungie can even announce when it is going to happen, and that it is a one time event.
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Or perhaps just a Survival mode that you can hop into with other players and hold off waves of enemies at a certain objective. That would be a good compromise. It would kind've be like firefight in that aspect.