Halo 3

Halo 2's Plot Developments
It is revealed in Halo 2 that all other Halo rings in the galaxy have been activated and put into standby mode, and that only a device known as The Ark has the capability to shut them all down. Sergeant Johnson, Miranda Keyes, 343 Guilty Spark and the Arbiter ponder this in the Halo control room. It is inferred that this device was sequestered away by the Forerunners, probably somewhere on Earth in remote pre-history. The Master Chief, meanwhile, returns to Earth in a spaceship he stowed away on when he left Cortana behind at her insistence. The ship that the Chief is on is a Forerunner Ship and it is occupied both by the last Prophet and its AI, which Cortana said she had great difficulty with as it fought her. Both the Master Chief and the Prophet are probably trying to head to the location of the Ark on Earth. The stage is set for a climactic final battle on Earth to find and gain control of the Ark which has either the power to shut down or resume the Halo Installation superweapon firing sequence all over the galaxy and also on High Charity to re-defeat the Flood.

After the credits, we are treated to a small cinematic. The short movie reveals the omnipresent nature of the controlling intelligence behind the Flood - the massive creature that lives within its depths known as the Gravemind. He recites a verse:


 * Silence fills the empty grave now that I am gone,
 * But my mind is not at rest, for questions linger on.
 * I will ask and you will answer.

Cortana appears above a console; its tentacle reaches out to her, but she raises her hand to halt its grasp - "Alright, shoot." is her response. Interestingly, as the Gravemind speaks in verse, the iambic pentameter of this phrase suggests that it is annoyed or worried.

The Rampancy of Cortana
Cortana's agreement to answer the Gravemind's questions leaves open the possibility that Cortana may have become - or may be becoming - Rampant, and either giving some information to or forming some sort of alliance with the Flood which will join the final battle for control of the Ark.

It should also be noted that Cortana is not a biological lifeform but a computer program and would likely be unaffected should Halo actually fire. It is likely that Cortana is aware of this, so people should factor that into her reasoning.

Cortana going Rampant would also be in keeping with the tradition of other rampant AI's in Bungie games - ie: the Marathon series. It appears at least that Cortana has motivations that aren't entirely clear and may in fact be above and beyond her programming.

There are various references to Cortana's state of mind (rampancy) in the Fall of Reach, at the beggining of the section 'Two betrayals'. It is noted in this that she has spent a long time in a huge network - In keeping with Marathon's guide to rampancy.

Master Chief's Shields
In the previous two Halo games, the earliest actions that the player undertake in the guise of the Chief include recharging of shields. The Chief has not had an opportunity to recharge his shields since he left Cairo Station in Halo 2. With this in mind, it is likely that the Chief will start out with shields at less-than-maximum charge, and only regain the maximum charge for his shields once he returns to a human-controlled space and gets to a recharger, or otherwise hijacks a Covenant-made recharger for his use.

Another fan argues:

Actually, that is completely wrong. That device is used to "test" the chief's shields, to see if they are in working condition. In the first game, he came out of cryo sleep, and so they needed to test if the cryo had affected his shields at all. In Halo 2, Master Chief gets brand new armor, and since it is brand new, it obviously needs tested.

First Level
It has been speculated by many people that the first level may acctually remble the New Mombasa fight that was origanly planed for Halo 2, it is also belived that it will feature a mini lvl of Master Chief taking control of the Forerunner artifact and having to land it some were near the battle going on at New Mombasa. [or possibaly link up with Frederic and learn the wearabouts of the other Spartans]

More Spartans
The possibility of the other Spartans return is also highly possible. There is a chance that Kelly will come back stronger due to her time with Dr. Hasley and the Forerunner artifact, as well as the fact that Linda, Will, and Frederic will fight by John early on or later in the game.