I was thinking of halo 4's firefight, and I thought this would be a good thing to have in it.
Wave 1: 5 grunts and 1 elite (X3)
Wave 2: 20 jackles
Wave 3: 10-15 Brutes
Wave 4: 10-15 Elites
Wave 5: Flood spores come and infect 10%-100% of the bodies depending on the difucutly
Wave 6-10: Same as above
Wave 11: Hunters
Wave 12: Buggers
Wave 13: Brutes
Wave 14: Elites
Wave 15: Pure Tank forms and Ranged Forms, plus 50% dead bodies reanimated
Bonus: An Endless onslaught of flood spores for thirty seconds.
There would also be a game mode where you have infinite rocket and jetpack, and are fighting flood spores in zero G which, i think we all agree, would be AWESOME!