It would be similar to the Armory or Armor Permutations, but it would be a set armor type of someone from the Halo universe that you couldn’t change. Kind of like the skins in Batman: Arkham City or the Elites on Reach.
For example, for Spartan characters:
Mark VI armor- just like Chief in Halo 2/3
Classic Mark V- like Master Chief in Halo: CEA
Mark IV Spartan- Halo Wars
Mark IV Gray Team
SPI Armor
Elites, if you can play as them:
The Arbiter- both Thel and Ripa
Shipmaster
Headhunter Spec-Ops Elites
Classic Elites- from Halo CE/2
and Reach variants- more or less- whatever you want to add or take out
And along with the player skins, you could also use your custom armor type from the armory that you made.
I know this may seem kind of weird or different, and you might just see a bunch of Master Chiefs out there, but I still think it would be really cool, and you would have to unlock them- achievements, commendations, whatever, not buy them.