What are they going to develop it into? It needs a reset
As someone who played Halo since the OG Xbox, to be fair I have no clue. Halo is in a very awkard position now. I felt that Halo Infinite was pretty much the reset that was handled very poorly ppst launch. It went to the old school visual art-style and level design which I thought was nice, but that was it.
For one, the game needs a different engine. Either UE5 or something else.
For two I never finished the campaign in Halo Infinite, I only played it for multiplayer so I have no clue where the story would go.
For three, I would just reboot the entire franchise if it was me story wise. Halo Infinite was confirmed to be basically Halo 6 because it follows the events after Halo 5 Guardians.
As far as gameplay goes I would be very ballsy and go the Mass Effect route but in first person.
Imagine a living breathing Halo world with your own squad mates. Some part of the human race and others from the covenant to help you on your mission. You would traverse different parts of the planet. You could have interesting dialogue and so on. Deeper background lore, cool side missions for a variety of characters.
The gunplay would still be classic Halo of course and the level design would be a mixture of both open wide spaces where you use vehicles to traverse and your typical interior corridors.
Multiplayer probably would just be same, no rpg elements or anything as I think that would kind of weird for a competitive shooter. So I am strictly talking the reboot here. Honestly, I think Halo should have a separate entity completely for single player and a different package for multiplayer. I feel like that's the route they're going to be going. Multiplayer always free, but campaign is a purchase.
Just my few ideas.