You really need to add in a theme of survival though. It should be a map like in ODST, the campaign city. You have full access and can roam around. If you begin to kill alot the covenant will be alerted and send in reinforcements. If you are able to procide by kiling them, they send in bigger reinforcements. It could get from a wraith to a simple scarab. This would add in a huge sense of fear.
You begin with a pistol and roam the city. You will come across small squads of grunts and covies.
You can gain weapons by going through buildings and picking them up.
As you continue through this adventure, you can pick up armor. Gain ODST and Spartan armor later. These would provide help by giving you a HUD and extra damage.
Objectives will be thrown in, such as taking over a building; evacuating civilians; etc…
This would become a story of its own, a story you craft. You create your story by the way you play.
At times and flood infestation would occur and you would be sent in to neutralize it.
I believe you should have full access to the city. I mean you can take trains to get across or cars. You can roam the rooftops and parachute down onto the battlefield. You can take over a phantom and man a scarab.
This would be a new mode called survival. A new and interactive experaince.
Possibly you could even call in AI to help you. If you find a radio, you could bring it along and it would fill your equipment slot. At times where you need help, you call in some ODST’s and they will come down in drop pods to help you fight through the enemies.
It would work on a point system. Each covie you kill is 100 pts. Each objective you do is 1000 pts. As you earn more points, the threat level increases. More covenant will be on your back. To lower the threat level, you simply get away from the battle or hide. (Thanks for Sircampsalot for the point idea and threat system)