So by a few articles, we have been told there are going to be things like Raids and Dungeons. For those who don't know what a Dungeon or Raid is, then I'll tell you. It is a kind of Co-Op level, mainly Linear, that you progress through with friends or randoms, and complete a common quest, while earning sweet new gear on the way. Now in World of Warcraft, these Dungeons and Raids were loosely tied to the narrative. I believe that Destiny's Narrative will be progressed through these Dungeons and Raids. They will play very similar to Halo Campaign missions. From what Bungie said in their Vidoc, totally unexpected things will happen, things nobody, not even the developers, would have expected. With Bungie's AI progressing further every game, able to make decisions on their own that effect the way the battle takes place. This is how Destiny's Narrative will be told, part by the developers, part by the players. The players will have their own "War Stories" because the Dungeons that tell the Narrative won't be 100% Linear. Like an RPG, the player will cause something completely random to happen that affects the entire mission, and not just a go here and press a button there. Tl;Dr: I believe that Destiny's Main Story Narrative will be told part by the Developers, part by the Players. The Developers use Dungeons and Raids that provide the backbone of the story, while the players play through and affect things as they go to provide the details. This is the Living Universe.