What would it be? Change weapon designs, add shield bleedthrough, and have speed,strength, and hitpoints lowered or hightened depending on if your an alien or human. And male or female. Also it'd be awesome to have a covenant slayer gametype that depending on the loadout, it would change your species. Like choosing a drone class would make you become a drone. Or a brute. It'd be pathetic to be a grunt though. REMEMBER: GAMEPLAY, not story.
[Edited on 12.22.2011 5:31 PM PST]