Halo 5 Guardians Speculation:
From the end of Halo 4, the voice of the Didact:
“…We are Forerunners, GUARDIANS of all that exist. The roots of the galaxy have grown deep under our care…”
That’s Chief in Forerunner armor in the box art, his body was altered by the Librarian, could be him in the type of armor that all Forerunners wore, because he is a pseudo Forerunner now, if not a Forerunner then at least maybe the mantle has been taken on by humanity, either way he is a Guardian.
Just a thought!