I would love to hear your Master Chief headcanon, mostly how he is autistic. Not in a "screw you, nuh-uh" kind of way but more of a genuinely interested way.
Admittedly, this is the headcanon I have the least solid evidence for, but here goes.
1) Master Chief clearly misses a lot of social cues in conversations. You especially see this in Halo 4 every time someone who isn’t Cortana tries to talk to him. Before that game, I coulda maybe just thought he was the stoic, silent type, but now I believe he’s straight up neurodivergent. And none of the other spartans you meet in the games (at least the games before 5 anyway) are like that, so I don’t think it’s because of the way he was brought up or socialized.
2) Master Chief has incredible focus. It’s one of his greatest assets, actually. Occasionally it gets him into trouble, as he gets so set on a goal that he misses the bigger picture (like how he almost sets off the Halo in the first game), but mostly it’s what allows him to just keep going on these crazy missions and stay clearheaded.
3) There’s no reason he needs to keep his helmet on at all times. Like, even between missions or during long travel sequences or while sleeping he has it on. Once again, this isn’t true for any of the other characters, including other spartans. So I figure he might be prone to sensory overload or overstimulation and his helmet/armor helps him regulate his sensory input.
Anyway, those are my thoughts. Once I saw it, I couldn’t unsee it.