I?ve read all books of Halo and not all Spatans are dead. Two Spartans (i think Kelly and Linda) and some Spartans from the Spartan III Projekt are still alive. They?re all on Onyx with Dr. Catherine Halsey in an Slipspacebubble which was made to protect them for Halo.
In the Trailer you can see a Allinazship. Maybe the Chief gets captured of the allianz and then some of the unsc will free him. Or he gets on the ship of the arbiter. But i think 343 Industries will make a great game.