It's master chief, Anon isn't a halo fan so he wouldn't get it. But it is

I mean, don't get me wrong, as a true Bungie lore fan, I think it's possible, but Master Chief isn't a guardian, and the god of Fate already has an important role in the story, why would it have two incarnations in the same story? The mysterious stranger is probably enough for now. They can bring Chief in near the end of the story to help tie it all together and really knock people over the heads with the tru7h.

The halo universe is completely different so that won't happen. But it's a master chief Easter egg that is confirmed haha

You are technically correct, but also wrong. Every Bungie franchise is connected, they're different pieces of the same story. Its why Bungie creates tens of billions of years of backstory into their franchises, to map out the connective tissue. They're probably parallel timelines diverging from the big bang, but they also somewhat emerge sequentially. The Master Chief is a incarnation of a god, called Fate, Destiny or Wyrd, a god in every Bungie game. The god in most of bungies universes is reincarnated as a hero throughout the history of the universe. In Halo it was the Iso-Didact, then Chief, in Marathon it was Yrro then the Security Officer, the mysterious stranger is Destiny's incarnation of Fate. The Librarians "Eden" / Catalogs "Edom" are earlier incarnations of the Black Garden, which also have a role in a particular piece of Marathons backstory, in " Kill Your Television" (see the expanded terminal), if you follow the clues you realize Kill Your Television is the ending of Halos story written over twenty years ago. Master Chief could technically pass geas off to the Marathon universe, so that a successor could be chosen by time, and could temporarily enter hypernation before moving to the Destiny universe. But you're correct, even if that did happen, this is just an easter egg.