Not sure if anyone has brought this up yet, but I was thinking about this a couple of weeks ago:
Remember the very first look we got for Halo on the Xbone? With MC wandering through the desert looking for what we now know is a Guardian? What if that trailer wasn’t for Halo 5 but was actually for Halo 6?
We know that Cortana has control of the Guardians with the intention to revive the Forerunner empire under the oppressive rule of the Mantle, and that Master Chief and friends are the only ones who are capable to stop her. In the trailer, Master Chief is wearing the cloak to most likely hide his identity, and now that we know the end of H5 it’s obvious MC is hiding from Cortana. I can only assume when Osiris saved Blue team from Cortana she wouldn’t just give up, she’s probably scouring every corner of the known galaxy looking for him. The Guardian shown in the trailer is probably Cortana’s, the one we saw at the end of H5, MC has finally tracked down his former companion and with the scene of him looking down at Cortana’s old storage chip it shows the chief’s resolve to put a stop to Cortana.
So yeah, just a theory but I think we actually saw stuff from Halo 6 before even learning what Halo 5 was about. (If anyone thought of this first please don’t hate me lol)