Hmm...something tells me you didn't stick around all the way after the end credits. Nor finished the game on Legendary setting. YouTube's your friend here. As far as the story goes, I only wonder if the next game will forego all of the previous Forerunner/Flood backstory and focus on some new threat. However, the very last scene of the Legendary ending hinted at the continuation of that backstory, given the appearance of a very familiar symbol on an object in that scene (visit Ascendant Justice for interesting articles that explain much of that backstory and the eventual fate of our beloved Master Chief Petty Officer).