A bit of clarification before we head into this, no, Chief will not be replaced by Thorne for Halo 5. It would be foolish to go that route.
With that said, by the end of Halo 5, I believe that Chief is going to be replaced and Gabriel Thorne is the likeliest candidate. I’ve had this belief for a good long time now and it seems to make people uncomfortable when I bring it up. I understand why, Chief is a memorable character and fan favorite. Fans have been with him from the expanded universe novels to the games and beyond. For better or for worse, Chief is what Halo is all about and it is inconceivable that such a huge character would be absent from the lime light.
Then again, people didn’t think 343i would kill off Cortana either and yet they did. Back to the point, however. Looking at the recent trailer, we see a Chief we haven’t seen, a man broken both physically, as the armor shows, and mentally, as his attachment to Cortana’s data chip shows. Everything Chief knows is starting to crumble, Cortana is gone, Halsey has been lost, his former teammates from Blue Team are still missing, at least to him.
Consider how much 343i seem to be investing in Thorne. The content in the Limited Edition belong to him, his background has been the most brought up among Majestic, Halsey recognized the potential in him and even Thorne himself seems to desire the truth about our controversial doctor. As time goes on, I believe we will see more and more of this character.
Tying this together are some threads left over from Halo 4. The Librarian did say that Chief and Cortana are essential parts of human evolution. However, I do not believe both were the ends of her plan. Cortana’s creation seems to coincide with the idea of the Composer. She was made from the living tissue of Dr. Halsey’s brain, the Composer turns living organic matieral into AIs. Though Cortana has perished, I see her as the first step to the evolution of human artificial intelligence. Chief is more obvious. The augmentations and surgeries he underwent are being spread to more and more people. The Spartan-IVs, as of now, are likely the pinnacle of what could be the next phase in evolution.
Finally, look at the one big overarching theme of 343i’s recent Halo developments. Sacrifice. In the Forerunner novels, Bornsteller had to sacrifice everything he has ever known to become the Iso-Didact and in time he would sacrifice the galaxy to halt the Flood. Chakas would do the same and become 343 Guilty Spark, assuming a new identity, losing his past life and ensuring the Forerunner’s plans continue. Halo 4 continued that trend with the demise of Cortana, she scarified herself so that Chief could live.
Now let me try to collect all these loose ends and threads and knit you a better picture. Chief and Cortana are the beginning of humanities ascendancy, paving the way with their contributions to the digital and physical realms. However, they are merely stepping stones, Cortana’s death has hastened Chief’s likely demise as well. However, a new generation of heroes has arisen since then and one of them, Thorne, shows promise.
It may not be satisfactory to some and scary to others, but given the theme of sacrifice, it may be the next step. The reclaimation is at hand and it isn’t going to be pretty.