I actually agree with the emp part because I think it's the part where chief is on the ground and the phantoms fall, and in the e3 trailer you see the same setting with a phantom crashing, and chief is in a host, so it could be another vehicle end mission, but since the missions will be longer, I think that the first half will be vehicular, and the last part is all out war, the battle with the Didact, and then an ending