Simple ideas for the Halo MMOFPS.
You pick either Covenant or Humans, you start off in training, and pick your basic class (In Covies, you pick your race and colors, etc...) In human, you customize your character's looks and start off in the Marines. You do training, make it through (you make it automatically) and are issued armor and a weapon. You are then shipped off to a planet to start off, you rise through the ranks,and in Humanity, once you reach the rank of Sergeant, you are available to try to join the ODSTs. Your stats must be at a certain level and you must have a certain level of XP. I'd imagine the game would work the best as a Morrowind-type game. You would then, at a pre-set level and amount of XP, are allowed to try to join the Spartans. You have a 10% chance of being chosen, but when you are chosen at that level, you become one after a bunch of work. If you aren't chosen, you can try again later.