I'm thinking Halo 6 will revolve around an explicitly random, bizarre journey with the unlikeliest of alliances Chief, (possibly) the physical manifestation of Cortana, and the Chakas 343. It'll be Halo: Primordium all over again.
The ending would show the Chief saving the Greater Ark from an impossibly monumental fate, with "Legendary" ending showing a choppy stop-motion cave drawings essentially retelling in an open-ended way about John and the end of that universe (like, when expansion destroys everything billions of years later...).