My Theory:
In the prologue of Halo 4, Dr. Catherine Halsey is being interrogated by an unknown representative of Oni. In this interrogation, the man inquires about the shortcomings and faults of Halsey’s Spartans - more specifically the Master Chief. As Halsey points out in this interrogation, Oni is looking to replace current Master Chief Petty Officer of the Navy, John-117.
This new Spartan on the box art of Halo 5: Guardians appears to reflect the Master Chief. I believe that we’ve just been given our first glimpse of the Spartan Oni has chosen to replace him, and it seems that this new character is on the hunt.
What are your thoughts? The Spartan has already been confirmed to be a man, and a new character to the series. It would be exciting to see a rival Spartan match up to the Chief.