Fascinating stuff.
From a story point of view 343 has made a big deal out of making sure that all it's media intrests concerning H4 tie together, even going as far as rewriting some of The Flood and Fall of Reach books. I would think that making sure everything is canon in this way is for the purposes of continuing the integrity of the Master Chief's story and not allow any holes in future plot points. Having said that, let me say this.
In my humble opinion, Cortana's key dialogue, the Prologue, the end cutscene and the Epilogue lay the Reclaimer story bare for all the world to see.
Assuming that 343 doesn't pull any manufactured plot twists, the Didact, having fallen into the Composer's stream and subsequently getting blowed up with it, is dead.
Also assuming that all media serve the Halo games storyline, Cortana as we know her is also 'dead'. Having given her all to protect Chief and safeguard Earth.
This leaves us with the leading in-game dialogue. Remember in the Sixth Sense, how in your face the plot twist clue was? "ic teh ded peoples. They don't even know they ded"?
Cortana: "They'll pair you with another AI, maybe even another Cortana model." I imagine breaking in a new Cortana would be enough plotline for an entire Halo game installment, much less two.
Halsey: "blah blah John is the future of mankind as a species." He's been evolved. So much so, that his bio readings are off the charts. He will be the tip of the spear when it comes to the impending doom that is the Flood. I also think that the legendary show-117's-face ending is an indicator of how much more H5 will be about John confronting/recconnecting with his creator, Halsey. 343 has gone so far out of their way to put their mark on the Halo franchise anything is possible. Who knows? maybe a helmetless Chief is in our future just like Mass Effect's Commander Shepard.
To support these ideas, I have to go outside the game. The Terminals are all about how humans fought the Forerunner almost to a standstill and presuming to uphold the 'Mantle' while defending against the Flood. They also showcased what a pain in the ass the Flood were to defeat and how much the Librarian cared for us.
Although The Composer did a bang up job at DE-composing, it only seemed to have initial success when it came to actually composing the people it digitized. So no matter what the Spartan Op's cutscene says I can't imagine we'll see the actual Librarian.
However.
Halsey's old spirit is the Librarian, Chief's is the Lord of Admirals, New Cortana will be the evolved ancilla, and the enemy will be a vengeful Precurser and their instrument of destruction, will be the Flood. Not sure where Spartan Gabriel Thorne fits into all of this, but I foresee him as a key player in the series.
Perhaps in between h4 and h5 in addition to more Spartan Ops we will get:
An ODST-like,The Chronicles of Kilo Five
Commander Sarah Palmer's Halo Wars 2, Return to Requiem starring Team Majestic
and a point and click flash game called Forerunner Legends: the Libraran's Story.
Whatever happens I'm all in.