To add to my theory, yes, I’m referring to them becoming the “Gaurdians of The Mantle”.
It would make more sense to have multiple “species” hold this title, as shown from the Forerunner, they ended up taking it to the extreme and protecting mainly THEIR well-being.
By having several different species, it would work more as a democracy, concerned more for the overall well-being on the galaxy and not one particular species.
Perhaps Cortana knew this and somehow gave John the coordinates in which we see him arrive at in the first “Halo” teaser for Xbox One. Notice he looks at her chip, looks up at the thing that’s just emerged from the ground, grasp her chip as if saying “Yes. I found it. Thank you”. I assume he had a sort of idea what he was looking for, for he did not seemed shocked nor threatened when it emerged.
However, when he was scanned by the Didact in the first part of Halo 4, he’s confused and “on-guard”, even when the Cryptum releases the Didact, he jumps to safety. Sensing the Didacts intentions.
The Arbiter could possible be a Gaurdian as well as Cortana. We know that the flood were either created by the PreCursor, or ARE the PreCursor in what they turned themselves into.
John, The Arbiter, & Cortana ALL 3 have interactions with the Gravemind and he didn’t assimilate them. However, he had no problem assimilating other humans, elites or even a Prophet & an installations Monitor (2403 Penitent Tangent).
We KNOW we haven’t rid the galaxy of the Flood. Hell, we blew up the Gravemind. Not even anything near as much as what the forerunner or pre-historic human race did.
Perhaps, the Flood (Possible PreCursor) simply left because the threat of the Halo Arrays had been stopped, had they been fired again, everything would have died, except the Didact as he was kept safely in his Cryptum.
The Flood, if they are the precursor, were simply making sure these “Guardians” came to be, and would rightfully take the Mantle from the Forerunner.
The PreCursor realized that a single species holding the mantle was a bad idea, and set this new “Democracy” in motion, perhaps unbeknownst to the forerunner, even using the forerunner to plant seeds to lead to individuals such as John.
One species will, and DID, become power hungry and even states so on Terminal Three in Halo 4 (Level: Infinity), this is said.
Didact: “The humans were not meant to protect others. The Mantle of Responsibility for all things is ours, and ours alone.”
Librarian: “Yes, the Mantle is ours, and we hold it so tight, even in death we shall hold it still.”
Proving the Forerunner held the Mantle so high, that, they, and only the Forerunner shall hold it.