With this Hunt the Truth advertising with ONI, this makes me think the organization might have a role in the game. ONI want to keep Humanity safe from possible threats. They might see Master Chief as a problem as he has gone AWOL, defying their orders. While you can’t fight all of them directly, I think they will convince the UNSC that Master Chief has become a traitor. Do you think ONI will be the antagonist in the game and Locke will turn on them and join Chief?
Probably a secondary antagonist with something much deadlier lurking in the shadows of deep space.