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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Perhaps you could fight your way through the city to a particular area or certain checkpoints and defend key areas of the city, the tower would have to remain a safe zone but I don't see why the outer limits can't be flooded by hive or fallen once or twice a year. Could become pretty intense seeing civilians running through the streets, chased down by thrall etc. I dunno just a thought