In addition, 343 has taken the liberty of creating the ‘ONI Files’.
You know how inaccessible the Halo 3 Terminals and Reach Datapads are to read in the games and on Halopedia? Remember the Conversations From The Universe? Remember the Eleventh Hour Reports prior to Halo 4, or the Cold Storage Reports from 2008?
You can read them all (and more) in a reader-friendly format here.
Now for the notes:
Fireteam Osiris was originally going to be Fireteam Majestic from Halo 4′s Spartan Ops/Escalation.
Kodiac, Zodiac, and Artemis were alternate Fireteam names for Osiris in the early stages.
Osiris began with three ‘blank slate’ Spartans - Spartans Smith, Jones, and Brown.
‘Spartan Smith’ was conceived “the child of James Bond and Lara Croft”, an ONI hitman assuming the mantle of the hero. As you can probably tell, Smith ended up being Locke.
Smith/Locke spent their military career making sure you didn’t know who they were, they’re explored through their actions and the way they spoke to their teammates.
Even thought Majestic was cut in-favour of Osiris, Thorne was originally planned to be the 4th member of the team. He was meant to be the opposite to Smith/Locke - less cynical, and an established character.
Sarah Palmer was originally going to be a member of Fireteam Osiris, the cast would have been comprised of: Smith, Thorne, Palmer, and Spartan Jones (who would be a Spartan fluent in alien languages).
‘Spartan Jones’ became ‘Spartan Vesper’, who later became Olympia Vale.
Meridian was what got Sarah Palmer replaced on Osiris, they wanted to give Palmer her own separate story and have a character who could serve as a window into a glassed world - so Spartan Song was conceived, who went on to become Holly Tanaka.
Ethan Peck, the actor who plays Thorne, was unavailable due to a conflicting project, so he was cut from Osiris.
Bringing back Nathan Fillion as Buck was originally proposed as a joke, but by magic, his schedule fit with Halo 5.
A member of Blue Team was slated to be one of the members of Osiris, but the idea “never felt right”.
Onto the Q&A:
In Halo 2, during the scene where Chief gives the Covenant back their bomb, the UNSC Marathon-class cruiser which gets destroyed right next to John is called the ‘UNSC Feeling Lucky’.
The Prometheans were still under control of the Ur-Didact in Spartan Ops, they were not actually under the command of Jul ‘Mdama - only working with him because they had a common goal.
SPI has been largely phased out by remaining Spartan-III operatives, but the photoreactive camouflage has been adapted to ODST armour.
Kitun ‘Arach, a Sangheili character who appears in Halo 5′s audio logs, canonically appears in the first mission of the game as the Sangheili General who you can charge the Promethean lines alongside. He went on to command Covenant forces during the battle of Sunaion.