There should be an "Enemy Invasion" random roll in Destiny. Where you'd be playing in the crucible and then the announcer would come in and say to everyone "The enemy arrives. Work together guardians" and the team separation would disappear and everyone would have to try and stop the massive enemy incursion. It wouldn't happen all the time but once in a while and it would be epic
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Bump. Similar to the event "the enemy is moving against each other" where the enemies begin normal, but become insanely tougher as time goes on.