This is possible, very possible. If not start like normal public events start while you're patrolling and such, it could be a random notification on the map, like the green ones at the tower, but on a random planet. Select it and it loads you into a game with others and gives you a heads up on what's going on like every mission when little light talks. Could be like a fallen ship has crash landed carrying guardian prisoners, and they keep coming in drop ships to prevent the rescue, make like objective based not its guard ghost. And eventually another faction comes in. For exams the hive come in to try and kill the prisoners or capture them theirselves for the light. Idk, but your idea for massive public events would be so much fun.