Once Microsoft set Bungie free and created 343 Industries, fan theories focused around either a Combat Evolved remake or Halo 4. Turns out we got both.
Ever since the end of the Halo 3, fans assumed that the mystery planet would be related to Forerunners. Many also correctly guessed that it would be a shield world like Onyx.
Also, given the Chief's penchant for being a walking deathtrap button for any Forerunner installations that he sets foot on, fans were placing bets on when it would be destroyed. In the end of Spartan Ops Season 1, Requiem does indeed get thrown into its sun.
Jossed: Fanfics upon fanfics had been written after Halo 3 centering around Cortana finding some kind of cure for her rampancy, such as gaining a body or being transferred into some kind of Forerunner supercomputer. Halo 4's answer? Reality Ensues and she dies from her rampancy.
A large amount of those fanfics also involved her becoming evil and opposing John. Aside from a few sections where she goes mad and delays the mission, she never turns openly against John and remains your ally until the end of the game.
Many posts speculated on the Precursors being the game's new enemy, when it's actually the Forerunner Prometheans.
Prop Recycling: Of a single-work, single-level example, though it is so subtle that it is practically unnoticeable. In the level Reclaimer, the Mammoth includes an amount of textures and geometry shared with other human vehicles and weapons, only exploded and scaled. For examples, the wheel mountings share color-shifted textures with the Warthogs, and the hand rails in the interior catwalks use geometry from battle rifles. This was not done out of laziness, but necessity, since the level was just so big that they had to use these ways of stretching the memory budget as far as they could due to the finite memory of the Xbox 360 console.
Stillborn Franchise: Not Halo itself, but Spartan Ops, which never progressed past a first season. Overall reception to the first season was lukewarm, with much debate about it replacing Firefight, its level design, the storyline, and the lack of Master Chief. Doesn't help that the story was written into a corner by the destruction of Requiem. Other comics have progressed the story of Majestic Team, including killing DeMarco and promoting Thorne to team leader.
Word of Dante: The game's article in Official Xbox Magazine claimed the new Covenant splinter faction was called "the Storm", but this turned out to be a mistake stemming from the lowest ranks being called "Jackal Storm" or "Elite Storm", etc. In-game, the faction is only referred to as "the Covenant" as before. However, many have taken to calling them "the Storm" anyway, to differentiate them from the previous empire.
Originally, new episodes of Spartan Ops would have been accompanied by a stylized motion comic instead of the current photo-realistic CGI cutscenes.
The Mammoth was originally conceived as being a much more literalBase on Wheels in its original concept as the "Tortoise" deploying into a siege-mode with bracing pistons, a raising sniper nest, and a deployable landing pad which accommodated a Falcon.
Concept art reveals that Drones were at one point considered for inclusion, including a sniper variant.