I was thinking that the Quest could start off at MDPL-13 power station, where a Master-Cheif look alike fights off the ghouls in the station, having information that these creatures turn into the dreaded flood. You go to various locations, eradicating ghouls, until you ru into the Arbiter. You can either help the Arbiter kill Master Cheif or you can kill the Arbiter for Master Cheif. If you defeat the Arbiter, you could loot the Arbiter's Plasma Rifle, an automatic version of the gu using Alien Power Cells. If you kill Master Chief, you would receive Master Chief's Assault Rifle, an assault rifle with a extremely high rate of fire. Also you could loot one of the two for stealthy Arbiter armor or the tough Spartan armor. Please expand quest with your own ideas
The G.E.C.K. makes this very possible and there are proboly ways to upload it from the pc files to 360 but so far thats not posible with PS3. but so far from info you already said. And not the ghouls. the geck you can take a chareter (thats proboly spelled wrong) and make a new base ID from an exisiting charecter. And dit that charecters fetures.