Here's my conspiracy theory...
First, I believe the sudden change in visuals is a move to introduce a VR in Halo games. Making the character design "more simplistic" reduces the likely hood of motion sickness (Microsoft has also put a lot of money into augmented/virtual reality)
Second, from what I've read Masterchief is in the game and this takes place after Halo 5. However, the spartan we see on screen either A: isn't Masterchief B: is Chief in a different set of armor.
Third, the presence of crumbling non-forerunner architecture suggests this ring was previously (or is currently) inhabited ruling out the new ring discovered by Spirit of Fire while suggesting this is Installation 07. (I didn't take the time to translate the radio's morse-code but it allegedly states "SOS ZETA HALO GRD")
Lastly, I would argue that Halo is intended to be a story. The gameplay has always accompanied cut-scenes rather than cut-scenes accompanying gameplay. Meaning, each mission has a predefined ending, its only how you get there which is up to you.
This is most easily described with Reach as an example, Kat tells you destroy the Covenant anti-air or get the UNSC anti-air online. To complete the mission you must do both, the order doesn't matter though. If Halo 6 is going to be "openworld" I imagine they would be taking this approach, using the illusion of choice. I'm not expecting, nor hoping for Elder Scrolls: Halo