The games campaign would explore both human and covenant military branch options. For the humans you would start out playing as a simple marine. The weapons are the same and have the same customizable option, but the characters and their customizations are unique and different. After the marine would be the ODST. Spartan would be after that. Next would be a power he suit. And end it with that. For the covenant you start out as a Grunt. Next the Jackle, and then the Elite. Last would be the hunter. For a DLC they would put the brutes as a playable class to eaqual the Elites. Add a few new guns as well as a few more maps and you would have an amazing halo game. You can even imply each class effectively in almost any online multiplayer mode. (I got the idea from the movie Halo 4 forward unto dawn.)
Might wanna talk about your thoughts in the Halo 6 wishlists. Links in my signature.