Here’s a pitch for what could occur in Halo 4 story:
The opening is almost the same as the trailer, where the Chief wakes, and evacuates the ship, he jumps ship when it gets into the atmosphere(similar to Halo 2 end/3 beginning) and crashes. Hie awakes to see Flood approaching, and with no weapons, he cant hold his own. As they are about to take him, A team of surviving SPARTANS rescues hi. He is told that these SPARTANS were deployed to Earth, but on the way they encountered the covenant fleet and were shot down. They crashed landed on this planet. During the events of Halos 1,2, and 3, the Flood arrive on that planet in order to scour it for life. The SPARTANS have been working to survive since. Once the remains of the Dawn land on the planet, they hope to salvage some kind of power and radio to send out a signal. So you fight the Flood to get too the ship, and you send out a distress signal.
Meanwhile, The Elites are fighting the Brutes and the rest of the covenant species. During space combat, the Arbiters flagship intercepts the Chiefs distress call. The Arbiter, hoping it is MC, orders his forces to go to the coordinates the signal came from, but the Brutes pursue. Unsure if he will be able to get to him, he sends a message in a delivery pod thing, to earth, to alert the humans of the signal, requesting assistance to investigate it. The Arbiter decides he can’t hold out long enough for human support to arrive, so he and the surviving crew take phantoms as their ship is destroyed. Once they arrive, they find the ruins of the Dawn. The Elites search the ship for the Chiefs body, but do not find it, meaning he must be alive, and on that planet. The Arbiter searches for him, eventually Reaching the forerunner city in which I think the SPARTANS would be located. While searching, Arbiter discovers a suspicious object. Unknown at the time, it is instructions the Forerunners made on the construction of the Ark, in case the Flood damaged it before it was used. The Arbiters instinct tells him to take it, so neither the Brutes, nor the Flood can find it. The Brutes, who tracked the Arbiter, land and Combat ensues. As the Brutes are about to overrun the Elites and Spartans, the Humans arrive.
The Elites and SPARTANS are evacuated. During the retreat, the Arbiter is shot and wounded. He tells the Chief to leave without him. So the Arbiter is captured by the Brutes, and the artifact taken. The Brutes are able to discover that it is instructions to the Ark, but a part of the data is missing, and is stored on installation 05. So Halo 4 ends as a race between the Brutes and the human elite alliance to 05, with the Flood not far behind…
I know that there are plot holes, but its just an idea.