Not forcing us to play as random characters we don't know or care about just so they can sell more novels, toys and comics is a good start. Additionally, they could also not cut so much content from launch that keeps people hooked beyond just multi-player. Where is the campaign scoring? Campaign theater? Firefight? Spartan ops? Do you know how much of a chore it is to replay campaign? They haven't even given us things like rally points to skip the early slog of the campaigns just to fight some of the good battles. There is so little to do as a single player.
Though I think they should expand on the cutscene and talking missions. Makes no sense why we have to jump from level to level with just a small cutscene and loading screens? Why not have us return to Infinity after each level, get some cutscenes, reflect on the mission, talk to characters, maybe pick up some extra objectives? 'Hey Chief, we're getting hammered by jackals with Snipers in this level, if you get a chance, can you kill them with headshots so they know not to mess around with us?' or 'Hey, I heard the Covenant are using th refuelling towers to keep their pelicans flying, maybe once you're on the ground and see them, you can destroy them and taken them out'. Just stuff like that. Changes up the monotony of the campaign.
If they want to keep the coop focus. Make us play as chief. We don't care about Jameson 'I'm not Chief' Locke, or the rest. Make the new team Chief, Locke, Arbiter and Palmer and then interchangeably add people like Linda for levels where you need a sniper etc.
I don't know, they did a lot right with the game. What do I know. They should just make what they want. If they don't listen to fans, the game sells but drops off. If they listen to fans, the game struggles to sell, based on the NPD threads. They know better than us at this point I guess.
