I think you taking part in the quest or mission could add towards a 'tally' of people who also have done so. Once so many finish an objective the next could be unlocked. There's many ways to go about this system, so it's difficult to say exactly which will be used. Past missions [i]could[/i] be always playable, but then how would that fit into the storyline? It's difficult to say. The way I personally see it is there will be two separate types of quests. Like I said in the OP, one would be of discovery, and introduction into the world. The other would be time-based quests, much similar to the weekly challenges. However that's purely speculation, and my personal take on it is constantly changing. Since I wrote this theory [b]a lot[/b] more has been said about constantly updated quests, and I'm certain they will be in the game in some way.