If you really want to stray from the yellow brick road and you like to climb, here's a looong TransAm side trip I took after dropping out of a 1983 Bikecentennial group tour called "Great Parks Odyssey" (Durango > Jasper). I continued to Jasper solo.
https://tinyurl.com/24e6wxubGravel from Grant MT to Tendoy ID. Gravel again from Shoup ID to just south of Darby MT. Rough in spots but I rode it on a loaded touring bike with 27x1.25" tires, pushing their limit. No flats, no damage.
pros? remote and scenic with paved stretch in the middle for services, Lemhi Pass, Lewis & Clark history, Salmon River, overnight at a fire lookout (?)
cons? bypasses a fun stretch of easier traditional TransAm. Maybe not rideable for all (as John requested). I rode this 38 years ago. I don't know its fire history and can't speak for conditions today.

correction: I was indeed using the fat, cutting edge 27x1 3/8" tire size that John mentions below. The wheels with mounted tires are still hanging in my cellar!