: My headcanon:
: - The Forward Unto Dawn is still in its H3 form.
: - Halo 4 Charon-class frigates are a redesign of the ship class by the Navy
: after 2552.
: - Chief's underarmor doesn't look quite as futuristic as it does in Halo 4.
: - The ODST O'Brien who was killed on impact during "A Day at the
: Beach" was the same O'Brien from The Babysitter in Halo Legends.
: - Chief chooses to only use the BR in burst-fire mode for combat efficiency
: - The Covenant in Halo 2 are speaking Sangheili when not in the presence of
: humans. Maybe also Halo 3, in some cases (imagine Arbiter's revenge
: against Truth in Sangheili; that explains Chief's lack of a response to
: the event)
: - Cortana is still purple in Anniversary
: - The Harvester is a Lekgolo-made vehicle
: - The new UNSC symbol is only on newer ships
: - Every Covenant variation across the games represents a different
: subspecies, much as humans have different races, and outside of the games
: all varieties can be seen together

: I'm sure I'll think of more as soon as I post this.

: What's your headcanon?

I share many of your viewpoints, save that Anniversary simply doesn't exist. :)

Maybe the best way to represent my headcanon overall is to view things with an accuracy rating - how close it feels to real 'Halo' for me. Here's a guess:

Halo: Fall of Reach 95% (comic adaption is 30%)
Halo: Contact Harvest 90%
Halo: Cole Protocol 75%
Halo Wars 60%
Halo: Bloodline 60%
Halo: Helljumper 80%

Halo Reach 80% (or 90% if I ignore the time stamps)
Halo: A Fistful of Arrows 95%
Halo: CE 100%

Halo: First Strike 75%
Halo Graphic Novel 90%
Halo 2 100%
Halo: ODST 100%

Halo: Ghosts of Onyx 80%
Halo: Uprising 10%
Halo 3 100%

Halo: Evolutions 75%
Halo: Legends 50% (legends/stories that might be based on some true things)
Halo: Forerunner Saga 50%(same as above)

Halo: Karen Traviss novels 10% (I gave away my copy of Glasslands and am not interested in reading any others - a depressing 'first' of mine regarding the Halo Universe)
Halo 4: 25% (Elites are good guys, Forerunners are dead and gone and don't look or act like that, almost all visuals different, Cortana's still kicking, UNSC is still noble and full of heroes)

Destiny 50% (...huh?)