This is just an idea but hear me out, Halo is a vast universe with a story that stretches over many years; I was thinking about a Halo RPG where you start as a normal UNSC Marine and have signed up just before the human civil war. This part of the story alone could branch of into many directions and leave open a whole load of opportunities. I was thinking of a Fallout 3 kind of leveling and perk system; you could go out do your mission and while killing things you could earn XP, however I also think that just like bullet storm you could gain more XP for ‘better kills’( though not to the extremity of bulletstorm) E.g. you could get 5 XP for a normal kill - an assault rifle spraying into the torso of the enemy, 8 XP for a grenade kill or 10 XP for a battle rifle/DMR headshot.
Going back to the Fallout aspect as the story progresses I was thinking that your Marine could receive Special training to become an ODST much like the way you receive power-armour training in Fallout 3. This would probably be a fair way through the story and would happen just before the Covenant begin to invade putting your guy on the front lines.
One last thing this would strictly be a first person shooter, I wouldn’t want halo to entirely turn into something it not. (I am aware its not an RPG.) so this would kind of take to the road that Borderlands went to.
Let me know what you think (everyone’s opinion is valid) and I will add more info when I see the need or think of some more stuff to add such as a story background.