User blog comment:NeverToWalkAlone/Halo 5, thoughts?/@comment-26194145-20130610212125/@comment-4207461-20130610214523

Chief is obviously looking for something. Could be that he is hunting down the Didact, and that this teaser may be somewhere in the middle of the game.

(Can't say if this part will actually be part of the game since it is just a teaser to say: "Hey! New Halo! More details later on!")

Or... he is finding a way to bring Cortana back.

Which that would be a letdown if the game is all about reviving your dead hologram girlfriend...