Wouldn't it be absolutely awesome if say every Friday on a rotating time (for the different time zones) the city was raided by different enemies so it would be like a huge fire fight and there would be an alert when it was about to begin and you could just join. There could be a special bonus and if someone got through there could be some kind of punishment but I don't no what it could be.
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Something to that effect could be amazing. If Bungie plans on having this game make us keep coming back to it, then they'll need some interesting things like that.