I remember reading in an interview long long ago that King said that his goal in The Stand was to turn the world into a Fantasy world like Middle Earth to set up the Quest in the second half of the book and the disease was just a means to that end. For him it was all about the Supernatural elements.
Apologies BTW - it had been so long since the thread started that I just quickly checked my initial prior post before responding, and repeated the error that it was an anthology film, rather than a book.
So, modifying my prior post, I hope that they don’t touch the existing characters, and just do other stories that are concurrent with the existing pieces. The problem then is, nothing that’s written can change the end - so like writing a prequel, there’s always going to be constraints on the story. If I were one of the writers, I’d absolutely focus on areas outside of realistic travel to the US - what do you do if you get mystic visions but there’s no longer feasibly intercontinental travel to get to the sources of your visions?
Or did Flagg and Abagail have franchise options? 
I’m pretty sure it was mentioned (during one of the Flagg chapters?), that it’s likely there were others like Flagg in other parts of the world.
I suspect it will be like the vignettes we got in the expanded edition set both during the plague and shortly after which were some of my favorite editions to the book (I prefer the original overall except for these).
If they did expand on existing characters it would probably be the minor ones. I could see an interesting story told from Julie Lawry’s point of view for example. Or the guy that got the power running in Boulder. Lots of minor characters seemed to have stories to tell.
Edit: realized I kind of repeated myself from the OP but oh well 
I’ve had this on pre-order since last month and I’m stoked.
It’s time to read The Stand again, but my grandkid borrowed my kitchen-sink version, and I don’t want to read the short version before delving into this new book. I need to request a book return.
My guess is that’ll be the bulk of the book. Stories like one King hinted at towards the end of It, when a minor character was leaving Derry and his “journey on the way out of town could be its own story”. Although I am hoping for some crossovers. Another story I’d like to see is one in which Ben Hanscomb from It meets Mother Abigail. Since they both live in Hemingford Home, it would make sense if they’ve met at some point, or at least know about each other.
Which is exactly why I avoided reading it for so long. When I finally did (only recently, don’t judge me) I was pretty impressed by King’s abilities for post-apocalyptic, road-tripping story telling.
I was left wanting to read more about the world continuing to rebuild after the pandemic, especially after the tribulations of democracy started cropping up soon after the Boulder population grew past a certain number.
Bumping this because this was just released today.