i think that the halo four should start off where halo 3 left off, i mean seriously bungie people were jerks for ending the game like that and the legendary ending had Master chief floating in the ship off to to some dark planet.
i think the planet should be like a forerunner construct, with sentinels, and you know the near end of the halo 3 campaign? well once you kill the oracle the sentinels start to attack you well that should also be on the new planet. also i think that the first mission should start with a video that shows cortana waking chief, and once he is out of the cryo-pod they would make there way to a escape pod and he would shoot out of the ship when it enters the atmosphere. the pod would land right in the middle of a yellow grass field and it would be on fire. the reason for this would be because the ship crashed before chief landed, so once he climbs out of the pod there would be sentinels all around, and you would have to make your way through the field unseen by the sentinels or you can run through blasting them.
well that's what i think the Halo 4 should be like at the beginning