I'd imagine that you will be playing as Chief for the game, But rather than being assigned a set fireteam, You could choose who you wanted.
When you think about it, Halo 5's ending had Chief, Locke, Blue Team, Osiris, Palmer and Arbiter all together. It would make some sense if you could pick your characters in Co-Op and have each one play a little different and also host different dialogue.
I can't see why they would change the format established in Halo 5. Chief will still be with Blue Team, Locke with Osiris. I'd imagine Halo 6 will be somewhat similar to 5, with a bit of a throwback to CE. Maybe have Osiris and Blue Team working together on the new Halo. That would bring in all kinds of nostalgia for me
Id personally like to see an event at the beginning of the game where it separates the chief or whoever from the team for a more traditional halo experience but still not totally scrapping the hole fireteam idea it could still work cos if you wanted to do 4 player co op or however many people they could make it where however many people are playing get affected and get separated instead of just the chief so kinda like how dead space 3 was and more people play the more the experience changes.
Another idea is they could give us both fireteams who work together in like the same mission but chose diffrent paths or somehow they get separated i dont know how but somthing to just keep it at 4 plays in one area 8 players could work if they wanted both firetsams but id find it to be to annoying i just want my solo halo experience or atleast just one companion. Maybe even let us chose which companion from either fireteams or they could do it like reach where you experiance abit with everyone!
I would really like that 343 industries follow the HALO 5 pattern, i would just give the Chief more protagonist than to Locke, for me the ideal schedule would be having Blue Team having the 60 percent of the missions on the campaign and Osiris having the 40 percent of the missions on the campaign, and would truly love seeing the arbiter going out in the 20-30 percent of the campaign even if he isn't a playable character, but imagine having Blue team as protagonists, Osiris as a secondary group but also having a very important role and finally seeing the arbiter in various missions, it would be awesome!