I'd guess multiple Spartans as well. Probably will get to play as MC for a bit to see his side of the Reach battle and most likely Fred to see the surface battle, and kinda jump in between the two from mission to mission.
Since most of the Spartans die, it'd be kinda cool to play as a couple different Spartans and see how they died. The end mission would be as MC fighting to get on board the Autumn before they got the f*** out of Dodge...