Well that's kind of the point. 500 million years of back story have happened. I could never expect them to cover that in the game as well as cover the entire story. I believe Destiny is meant to be the introduction to this world. Establish that it exists and what races live in it as well as the conflict. They made it clear that Guardians are only resurrected to help the Traveler fight the Darkness. Why would they waste time explaining 500 million years of lost time when the fate of the human race is behind the corner? Now that we've struck back at the Darkness and shown that Earth still has the ability to survive, more opportunities to explain the story are opened up. We already know there are missions and a raid in the Reef. No doubt that will involve the Queen and her intentions. The story element I found is that the Vex have the ability to time travel. One can assume that it will be a factor in future missions involving the Vex as well as their intentions. If they can warp through time, why didn't they just eradicate the human race? Unless their temporal command is limited to smaller time gaps and stasis fields (Frozen Vex in the Black Garden).