I thought of this theory back on halo 4, but after watching the mcc cut scenes multiples times (the locke ones) it confined my theory (at least for me). For the longest time the idea was chief didn't have a personality so we could "be him" but now chief is developing his own personality and we even saw glimpses of his face. But now you might be asking who would "we be" the awnser, our multiplayer/spartan ops characters. In halo 4 we got spartan ranks and we were members of fireteam crimson. Who while heavily involved with the story of the unsc infinity never got a single line of dialoge or cutscene. However in the mcc locke cutscene we see Spartans in the background that we never Cleary see or hear from suggesting that "us" our spartan characters might be helping locke in the search for the chief. And lockes storyline will be at least partially in spartan ops.
Remember this is just a theory.
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I feel that the Chiefs role in halo may be winding down to make room for a new protagonist, 343 said themselves that can't have a 98 year old chief in halo 23 (paraphrasing of course).