A halo game where you a normal human are a space pirate leading both human and aliens on raids and contact jobs.
You could start as an abandon Marine trapped in a Covenant ship and you have to fight your way through as a regular human. When you take over the ship you have a small crew who where also prisoners.
You could have anything from Brutes to hunters join you, why not throw some rouge Spartins in there for good measure.
Have hige battles in space, and fight your enemy in the void.
Manage your ship, and crew, steal enemy ships. Use/sell, weapons, veichals, and gather more to your cause. It could be like halo wars a micro management game. But boarding party’s are fought in FPS.
Have a mutiny break out where you with kill or reason with the traitors. Have bonding mechanic where you hang out with your crew. Desine your own ship or buy/sell one off/on the black market. Introduce a planet made up of only pirates, as a central hub.
Customise your character, from marine armor to a modified spartin armor. Mod your weapons to your liking. Make yourself either a room, or crib.
There could even be events that you the player can compete in.
Like a flood event where you have to save as many people as possible, or a glassing event where you have to stop the covenant from glassing a planet.
Room for possibilities is almost endless. It could be called ‘Halo Void tales’ or smotging like that. Just a thought.