Spartan Locke - - YouTube
Master Chief - - YouTube
In both of these videos, one of the two is victorious while the other is lying in utter defeat. I believe that instead of this being an actual situation that will occur, it is an analogy of what both men have come to know about the other and the situation at hand.
Spartan Locke when victorious seems to acknowledge Chief being the Hero of days passed, but also mentions the conquering destruction that lies in his wake. He mentions that his mission is to save “us” from the Chief.
The reason I quoted the word “us” is because I believe Locke thinks what he is doing is a saving humanity, but is actually saving ONI. I am very confident that Chief finds Halsey and finds out what she knows about ONI, undoubtedly something dark, while ONI brands Chief a traitor and sends Locke to hunt him down (for actual reasons unbeknownst to him).
When the scales have shifted to Master Chief’s side, he asks Locke rhetorically if he found what he was looking for, and if he wanted it to end in the destruction of the city. Chief also asks Locke if he still thinks that everything he had done was worth the price. In the end, Chief says Locke’s mission is complete, and that his own is just beginning.
What I think Chief means by everything he rhetorically asks Locke is to prove a point that he (Locke) was fighting for reasons that he wasn’t aware of, and didn’t think the situation would escalate into the destruction of a city. I think Chief knows Locke has no idea what the real reasons of the mission were, and Chief wanted to get him thinking about what is actually happening. When Chief says his own mission is just beginning, I think this is the aforementioned bit about Chief meeting up with Halsey and discovering what ONI is up to, making Chief need to take ONI down. I think this is what Halo 6 will be about.
As for Halo 5, I believe It will be structured much like Halo 2, jumping back and forth between Chief and Locke, but mostly centered on playing as Chief. Chief will be working his way into uncovering what ONI is hiding, and Locke will be working on finding Chief and understanding what he is trying to do. Near the end of Halo 5 we will see ONI do something drastic to cover their tracks, perhaps something like bombing an entire city. Locke will blame the incident on Chief and brand him as a traitor, while Chief will open (or attempt to open) Locke’s eyes and show him what is truly going on.
This is all what I think is going to happen in Halo’s 5 and 6, I could be entirely right, partially right, or completely wrong, it is just my thoughts on the what the trailers actually mean/stand for.