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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I think within dlc and the tower being the last safe place on earth that's where the final raid will take place cause someone will attack it.... Orrrrrr..... [spoiler]the travellers light will fade and he will corrupt and he will be the final boss in earth raid. Be a good part to the story aswell[/spoiler]