If Halo was a film series, and this was your suggestion for its sequel, then I'd be fully behind it. However, if Masterchief 'isn't needed to win fights', then there would either be gameplay/story dissonance with teammates being generally useless in combat whilst you're being told how awesome they are, or, you find that you don't have to do anything in the fights in the game, as your super awesome supersoldiers kill all the enemies and advance for you. I'm just not seeing how much of a game can be created from this idea.