One big par of a mission with many, many, many friendly and enemy AI's. Kind of like a huge brawl. With many troops, light and heavy vehicles, and aircrafts. It would be cool to be involve in something like this. Fighting along your comrades and fighting a bunch of enemies who trying to kill you all. Many would die, many try. Think of all the fire and explotions.
The Halo series is basically always trying to pull this off. The reason it never has is that the hardware can't keep up with it. They could make a more stripped-down game with more ugly/functional graphics and less polygons, and realize a much larger battle that way - but demands are such that the games keep getting prettier to keep up with current gen titles, and thus the constraints on the number of things that can happen simultaneously in one area stay roughly the same.
Plus it is not realistic from fighting styles from the Vietnam era to today. We always fight in small groups in the modern world so why would we try to fight in a large battle in the future.
If the campaign was realistic in any aspect, why wouldn't the covenant or humans just use bigger bombs or even nukes for most campaign situations? What happened to current jet technology?