Before you start reading this, please consider that it might be a spoiler. Having been an avid fan of halo since its beginning and being an avid fan of the books that have been released, I have thought of how the halo story could be unfolding.
Let me start by saying halo 4 is a part of the halo universe and HAS lived up to the legacy left by bungie. That's my opinion. Stop reading now if you don't wish to be spoiled by what I am going to say, this is my belief of how the next halo instalments will be going.
For those of you who have read the books and played all the halo games, a notable character in them is the monitor of installation 04, '343 guilty spark' he was named by the iso-didact, bornsteller. He was once a HUMAN named Chaka. He was absorbed by the flood but rescued by the iso-didact who stored him within the monitor before wiping his memories.
'343 guilty spark' says to the master chief during Halo: CE, that he is forerunner. During the cutscenes when the master chief places the index into the halo's firing chamber. Guilty spark says 'you asked me once, would I fire halo , if it were my decision?'
Hang on.. Master chief wasn't alive during the forerunner era so why would guilty spark say that? Let me continue..
In the book halo: silentium, page 314, the Iso-didact bornsteller says to guilty spark, 'tell me chakas, if this was your choice, after all we have seen and survived, would you fire the rings?'
Not once have we ever seen the mast chiefs face, and in halo 4 the librarian, the Iso-didacts WIFE, causes a change in the mast chief to allow him to be able to defeat the ur-didact.
Does this change possibly accelerate the chief to become a forerunner? Did the librarian put in his genetic structure that seed? I will hold off on my further theories... For now