I vaguely recall there being some discussion about the possibility of there being a group of extremist survivalist dominating chunks of the Pacific Northwest. I don't know if this particular aspect has been brought up before, but if such a thing
were added, it would be a great way to reference
yet another classic post-apocalyptic work in much the same was as the Catholic New Rome being in Missouri being an homage to
A Canticle for Leibowitz.
I never read the book, but Eastern Washington is great terrain for a horse-based army like in the movie. Montana and Wyoming as well. Idaho's mountains would make it hard for a single horde to dominate both sides.
Two or three steppe style powers would do well. Also the region has some large indian reservations that could spice things up. Considering how tense race relations can get in our non apocalyptic times, I could easily see a couple of 'Cowboy' culture survivalist powers striving for control against a Blackfeet-Cheyenne alliance.
Religion would be tricky, but I could easily see an Americanist offshoot dominating the 'white' communities. People there tend to be highly, even aggressively, patriotic, even the ones who hate the government. I can see the plains Indians practicing a syncretic Christian and shamanist faith (quite a few already do). For extra drama, both could have the black hills/Mt Rushmore as a holy site. Give the Indians a decision to destroy it that provokes an Americanist holy war.