So with very little info to go for Halo 5 Guardians Campaign, I thought of a couple of ideas. First off the chief has gone AWOL for thee reasons. First off he has gone AWOL in an attempt to bring back Cortana. The second being he is going out after whatever or whoever is attacking the colonies. The third idea is he is going out to find Halsey again to find a way to bring back Cortana. Now Locke hunting down Chief to bring him back is probably orders from ONI not the UNSC. The reason why the UNSC want the chief back could be because he is the poster child for them, he did only save humanity in every game he stars in and not to mention what he did during all the books (which are officially cannon for those who don’t know). Now for those who don’t know Osman the head of ONI doesn’t like Halsey at all, cause Osman is a Spartan - II that didn’t make it through the augmentations. So odds are Osman sent Locke on a mission to bring home Chief alive, but if he was to resist it would then become an elimination mission. Locke and Chief will probably meet on a colony that is under attack and have to work together to repel the attack and then they go from there.
There are my ideas, what about everyone else? Also I’m sure i got some of the info wrong so feel free to correct me.