I like the Northern Tier route, too, but I wouldn't sell the Trans-Am short, at least the Oregon section from Eugene to Hell's Canyon. Quite a bit of mountain passes as well. No "four passes in four days" like the NT, but there is the "three passes in one day" section from Prairie City to Baker City. McKenzie Pass is great if you time it so you can cross it, and the Painted Hills country near Mitchell? I don't think there's anything quite like it on the NT through Washington.
So to complicate things, I'll throw out yet another routing option: Head south from Vancouver BC and pick up the Sierra Cascades route and take that until you intersect with the Trans-Am route in Central Oregon. I haven't done all of SC through this area, just parts (Sierra Cascades overlaps the Northern Tier from Sedro-Wooley to Twisp).
Options, options. A lot of things to think about.