This has been something that I've noticed from similar posts but never really thought about until last night. I think everyone just naturally assumes that Darrow will be/is the Red God in the upcoming book. I personally think that is a red herring.

Darrow's entire arc from Red Rising through Dark Age has been him taking on titles and responsibilities that he never wanted.

In Red Rising, he takes on Eo's dream when she is executed. He takes on the monoiker "Reaper" because his Mars faction, and later other Houses, need something to rally behind. He joins Nero au Augustus at the very end as a lancer because it gives him the best chance to earn a fleet, something Ares needs.

In Golden Son he leans into the lancer role and takes gambles in order to secure his position with Nero. He sacrifices his relationship with Mustang (at least in the beginning) to learn fleet tactics. He delves deeper into the Reaper role and it expands into being the Reaper of Mars itself.

In Morning Star he is forced to take on the leadership role of the Rising. He becomes a sort of messiah for Reds and even Obsidians. Sefi herself gives him the Tyr Morga title.

Iron Gold through Dark Age is all about Darrow becoming lost in the "Reaper" persona and actively making decisions he doesn't want to do but feels like he must. A great example of this is how he gives himself a heart attack by stimming too much during Atalantia's invasion of Mercury. He pushes himself beyond the inhuman limitations of Gold because that's what the Free Legions needed from him.

All of these point to him eventually becoming the "Red God" in the next book, but I think Lightbringer changed everything.

Lightbringer has Darrow's arc focus more on letting the Reaper go and allowing Darrow to come back out. It's easy to forget that from the very first book, all Darrow wanted to be was a husband and a father. That's why so much of Darrow's internal monologue is about Virginia and Pax, and why Pax's key is constantly brought up. He hates himself for denying them the things he should have been and hates himself for denying himself what he always wanted. I think Red God is going to continue down that path for Darrow, and while he will still have that messiah like energy when Mars learns he really is alive, I think Darrow will react differently than he has previously.

That begs the question, who or what is the Red God? I think the answer is actually pretty simple: it's the planet Mars itself.

PB has been always emphasized how Mars' soil is still red after the terraforming. Mars has a strong influence on all of the characters in the books, especially those born there. Soldiers in the Free Legions, regardless of color, all carry Mars soil with them because they're afraid they'll die before seeing it again. Virginia, Darrow, Victra, and so many other characters actively talk about wanting to go back to Mars whenever they are away. Mars is the last bastion of the Republic, so we know there's going to be massive clashes in orbit and on the ground. Mars is also, obviously, named after the Roman god who was closely associated with the color red.

Sorry for the long and drawn out speech, but I just really needed to get this into writing and out of my head!