So if you remember E3 2013 Trailer, we could see the Chief alone wearing a poncho, walking through a place which seems to be The Ark. He has his visor broken and out of blue a Guardian wakes up.
My theory is that this trailer could be for Halo 6, due to some facts:
• 343i never confirmed that trailer belongs to Halo 5: Guardians. Even the title only says “Halo”.
• The Chief has his visor broken, so it has to be after the brawl with Locke.
• All the Guardians we saw during Halo 5 Campaign were blue-lights. In E3 2013 trailer, the colors were orange.
• The Chief wearing a poncho. We haven’t seen him in Halo 5 Guardians with it.
• Alone in The Ark: In Halo 5 there is no moment where Chief was alone. And not talking about to be in The Ark.
Conclusion: I know it was just a symbolic trailer. But symbolic for what? The plot of Halo 5: Guardians has nothing to do with that trailer.
What do you think guys?
I think it was just a way to introduce the Guardian.
I think you may be looking into it a bit too much.
Nonetheless, you’re right in what you’re saying. But don’t confuse conspiracy with clever PR. 
Alright… So it was never confirmed that the halo e3 trailer was actually halo 5 so what if that was actually halo 6 ? Following the events of halo 5 and master chief’s visor being cracked by locke and the poncho ? We know that Cortana is controlling the majority of all Ai in the unsc and the guardians shutting down all technology is safe to say that the resources were pretty scarce that making the chief use the poncho former protection or disguise and that orange guardian? Medican bias … He was trapped inside a guardian on the ark