I think a halo MMORPG would have pretty good potential, but I'm not sure that it would end up being that good.
Also, how would they give players freedom? You'd be stuck doing whatever your superiors told you to. You wouldn't choose "alright I want to do this now, see this planet, etc." you would go and do as you were ordered (assuming you were a part of the military).
Beware the Australian Fridge
Veraam:
One with power = Corruption
Two with power = War