This story follows what happens to Cortana and the Chief after Halo 3. I have not read any other Halo 4 threads, so if any of my ideas sound like yours, then great minds think alike, I suppose.
-The game starts with the Chief and Cortana floating towards the unknown planet that we saw in Halo 3. They enter the atmosphere and crash onto the surface. Throughout the game, you discover all the secrets behind its' creation.
WHAT THE PLANET IS
-The planet is a Forerunner built world. It was built along with the Halo rings and the Ark. Like the Ark, it is outside of the Milky Way galaxy, and is out of range of the Halo rings. The planet, while artificially built, is completely self-sustaining, having the ability to grow food, create oxygen, etc. All the necessities to sustain life.
-When the Forerunner activated the Halo's, they left the galaxy en route to this newly built planet. The planet was meant to be a new start to the Forerunner civilization, a safe place that they could begin rebuilding and expanding on.
-By some unknown means, some flood still remained and managed to find a way to board their ship(s), hiding out until the Forerunner reached their destination.
-After the Forerunner made it to their new homeworld, they fell under a second attack. After their first battle with the flood, they were much more vulnerable and were easily overwhelmed. With the Halo's being out of range, and no other options, the Forerunner were overrun by the flood.
HOW THE CHIEF ENDS UP ON THIS PLANET
-The Ark has two portals, one leading to Earth, the other leading to this planet. The Halo's cannot be fired directly from the planet, so they have a portal leading to and from the Ark, in case they ever needed to fire them again.
-Reading the Terminal on Halo, you found out that Mendicant Bias was talking to the Chief. Right before 117 activated Installation 04, Mendicant Bias states that he made a mistake turning against his creators and he wants them to know he has changed, and the Chief will be his example. When the newly re-built Installation 04 fired, it heavily damaged the Ark. Right before the portal collapsed, Mendicant Bias changed the coordinates from Earth to the Forerunner planet. Thus, the Chief and Cortana go through the portal as the portal switches to the Forerunner planet coordinates.
WHAT THE GAME WILL BE ABOUT
-The Chief and Cortana discover that the Forerunner planet is infested with flood, and they are actually the infected Forerunners.
Because these flood are infected Forerunners, they are highly intelligent. Unlike previous forms of flood, they can think and work together without a central Gravemind. They can build, repair, modify and even invent new Forerunner technology. These flood present a fresh new challenge, as they are highly tactical in battle, using technology and outsmarting opponents in battle situations. Unlike the other games, these flood could be very formidable foes.
Throughout the game, you will be battling the flood infested Forerunner and their technology. You will have to become resourceful, adapting to the new weapons and technology, as no UNSC or Covenant weapons or equipment will be on this planet.
-*The original Forerunner technology knew what had happened to the Forerunner, and began to rebel against their infected creators. So throughout the game, the original Forerunner technology existing on the planet would aid you while battling the flood and their creations.*
-The Chief and Cortana's main priorities are to get off the planet, and if possible, eradicate the local flood while doing so. They devise a plan to highjack a Forerunner ship and use its' slip-space capabilities to make it back home.
-They also figure out a way to either eradicate the local flood, or destroy the Forerunner-built planet. (I have not thought of an interesting way for them to do either of these yet, so I'll keep thinking...)
-The Chief and Cortana manage to steal a Forerunner ship and make it off the planet. The game ends with the flood (and possibly the planet) being destroyed, Cortana setting coordinates to Earth, and the ship jumping into slip-space.
I do not take credit for this post, this was posted on many other sites other than this one.
Hope It Was Useful