“Flood Firefight.”
In this game mode your not a spartan your actually a marine, you won’t have jetpacks, spartan boosts ect. You and a full team of however many you teamates you want. Fight against waves of flood that progressively get harder to kill. This mode could have different objectives like defending the core from the flood or just surviving. Since your a marine you wont have shields either and will be very easy for flood to kill. Whatever point system they have next will be the system that gets you different weapons like the REQ Stations. Although it’d be very hard to get more weapons as a marine. It would also be really cool if after time of being a marine and doing well you can progress and become a ODST. One point even become a spartan but I honestly I think it should be a one time thing and only one can be a spartan like Master Chief or something. Once the spartan dies they respawn as a ODST or Marine.