I remember some prior story-arc ideas (like the one about Mason in Vietnam) and felt they all had great potential. The best of those, however, I am of the opinion are Honor's Suffering and the one about the mysterious blacksmith in Nevada...
I once started writing a fan-made Storybook arc (of three chapters) about a CDC agent trapped in a neverending Grief loop, though I quickly stopped due to realizing I cannot write properly a book as well as my vocabulary is limited. But it might be an idea.
I was never really a fan of the over-the-top, brutal, simple-minded type of hero soldier Dempsey is, but McCain is kinda the opposite. He is softer, yet brave, funny (in a way better way than Dempsey) and has emphaty. I think it is a character alot of readers can relate to, as he seems one of the most 'normal' persons so far (not in a boring way, though). However, as with all characters, I like to see him having a soft spot/flaw or something. Perhaps one time he chooses for his own life above someone else, as it is the nature of human survival (however I absolutely dont see him as an egoist. Perhaps something else is better fitting). Eventually, Peter will die, as only a few main characters do. I hope that at that time, readers really live in him and are really saddened by the fact he dies. If you succeed in that, you are a pro-writer.
Another thing: A lot of the lore about Broken Arrow, CIA, Cold War, etc, happens on the background (not in a map). Perhaps some of them are fitting in parts of Book 2. Alot of it can be discovered in the early chapters of Book 3, at least.