I'm only posting this because I'm surprise that Savage Worlds wasn't being talked about. Going with a general purpose system can give what you are looking for. Why I bring up SW is that there are a lot of setting books already made for it. Give it a look it's a simple and flexible system. I'm using it with the Cthulu book t run the Hellsing anime. I'm sure SW will easily adapt to the Halo flavor.