Covenant Ship

As revealed by Joseph Staten in the Halo 3 Legendary Edition commentary, originally in Halo 2, there was going to be a level after Cairo Station taking place on a Covenant Assault Carrier, which would have served as Miranda Keyes's main introduction.

Content
After opening the airlock you would be blasted toward the Covenant ship and board it instead of blowing it up with the bomb. You can easily see the whole ship as you drop. This level would involve infiltrating the Covenant ship, probably an Assault Carrier, taking on and killing its crew and eventually stealing a Wraith and end up destroying the ship by bombarding its power core with the Wraith's mortar. As mentioned by Joseph Staten, Commander Miranda Keyes and her Orbital Drop Shock Troopers would have also been onboard the ship in order to help you and may have played a big role in the eventual outcome. This level however was cut due to time constraints and replaced by a cinematic that created "just as much work" for Bungie Employees.

Trivia

 * This level would have probably revealed a lot of information about the Assault Carrier and its interior.
 * The Covenant Bomb was added in order to destroy this ship without needing the level and find a reason for The Chief to open the airlock.
 * Evidence of the level's existence can be found in some of the old Halo 2 teasers in which John is clearly seen jumping from Cairo Station without a bomb.