I've always wanted a humans vs flood game type. Where you have a team of Spartans locking down a region against flood combat forms. The combat forms have a jump that has a small cool down, and limited use of weapons( can pick up, but no reticle, and random hits), while the humans have to survive in the dark. Each combat form also has a small swarm of infection forms, which it can send out using the grenade button. Each spartan dead joins the flood, and you get a very tense, last-man-standing situation.
Maps would be dark, dingy locations, in a poor area of a major city, or a forerunner structure. Hell, we could also have a floor of the library. It would be difficult as hell to implement, but damn, it would be awesome.