thhe chain gun idea seems far fetched and useless, so I've removed it. it wold make much more sense to have artillery cannons, and a longsword flighter cover. marines manning AA turrets would be useful but I would much rather expect brutes and jackals and drones to come streaming across a field, and defenders fighting, when all the sudden the chief is forced to abandon the main defense force for another purpose. a giant battle would be to hard to coordinate in the game.
NO TIME LIMITS(unless in a warthog driver/gunner combat scene)!!! A general mission objective is much more sound.