a Halo MMO? that would never work, there would only be 4 classes(spartan, hunter, elite and Jackal sniper), and only one would be on the humans side, and the weapon classes would be pretty limited i think
MMOFPS, not MMORPG.
Look at something like Star Wars: Battlefront
or Battlefield: Bad Company
with a couple hundred real people fighting one another, or the upcoming Huxley for an example.
I'm sure with a bit of imagination one could come up with many different players classes for each side in the Halo Universe, you don't have to stick to the characters seen in the existing game, and you can give different specialities to individual classes (sniper and demolition and melee Spartans for instance). You could even play the Flood.