When I first saw the two trailers, the juxtaposition of Chief vs Locke as a hint at gameplay. In HALO 2, the player gets to switch between the Chief & the Arbiter. In Guardians I believe the choices will be similar in that you'll get to choose to play either as the Chief or Locke. Unlike previous iterations of HALO though, I think your decisions will have consequences similar to Mass Effect that will lead to differing outcomes in gameplay. How that would play out in future HALO titles I don't know, but it seems a likely possibility, though I'm not sure it would be an attractive option to be able to play the usurper as the Master Chief has been the focal icon of the franchise, but if 343 is willing to kill off Cortana, they might feel justified in shaking up the very foundation of the HALO universe and have other characters such as Locke carry the load in the future.