Whatever skill ranks they implement (hopefully Halo 3). They should award you special armor, emblems, achievements, avatar awards and skins for hitting certain Skill and Military ranks.
Speaking of Military ranks… it would be cool if you could choose your Branch or attached Branch of service for military ranking. Have like a little comeptition with branches for unlocking things like a small community event.
This will let in more rewarding customization and people to better themselves.