The Master Chief is truly the ultimate warrior. However… what if something were to challenge that?
Obviously, there are now Spartan 4’s in Halo. I would assume these new warriors would be genetically superior to the Chief based on the superior technology of this new future world. If there was a reason, for instance a civil war, for the Master Chief to fight these new spartans, I feel like it would make for an intense and fantastic boss fight if executed correctly. What if the Chief even had a specific Spartan 4 as his rival, someone who not only is genetically superior but almost equally skilled and experienced in combat? That would be so awsome! It would also be a good explanation for 343’s integrated fiction with multiplayer.
Given some good AI, a good battlefield, and a massively powerful foe with a good sense of awareness (You probably won’t be able to just sneak up on him/her), and the implied increase of speed and athletic movement of Halo 4, I would forsee 1 on 1 boss fights the likes of which Halo has never seen.