First off, before I say anything I have a feeling this Halo game will be much, much more better than the original Halo 4. Why? Because this time 343i has more experience with what the fans are wanting, plus they will have learned from any mistakes they made in Halo 4.
Now onto the theories. I think that this new spartan will come down to one of two different possibilities. Either this second spartan will be like Masterchief's right hand man, assisting him throughout the game to defeat more forerunner enemies. Or he could be like many people have said, ONI's new spartan hunting down MC. From that first video I saw of halo 5 ages ago, I just felt like MC was no longer working for the UNSC. Perhaps if ONI hired this spartan to kill MC perhaps MC is trying to eliminate ONI? What do you think?