Lot to see and do here in the PNW, if you come for a visit you might not want to leave. If I were planning a trip like this I would do as
@JLLT4R said, come into Oregon on 95 to the Alvord Desert/ Steens Mountain area, explore there for a couple days then head north to the Ochoco Forest. From there go check out the John Day Fossil beds and surrounding area. I'm sure a lot of this could be done by running sections of the Oregon BDR but I'm not to familiar with the routes. Once done in that area I would head for sections 1 and 2 of the WABDR, maybe 3. If you opted out of section 3 of the BDR you can take HWY 97 out of Ellensburg north to Leavenworth(section 3 consists partially of hwy 97). From there east to Wenatchee and continue north on 97 to Winthrop where you can get on HWY 20 (or keep heading North into Canada). Take 20 all the way west across an island and a boat ride until you hit an ocean. Leave yourself a few days to explore the Olympic mountains and peninsula, then follow the coast south until you're ready to turn inland and back to Vegas. With so many things to do and see in the NW you kind of need to lock in a route and then pick what to do along the way. I think I would end up zig zagging all over if I tried to cram everything into one trip.
A few questions that might help narrow things down for myself and others reading this.
What type of vehicle will you be in and how much HWY vs Off Road are you looking to do?
If you're going off road, what difficulty of trails are you interested in?
Are you planning to move daily or stay multiple nights at each stop?
For camping do you prefer primitive dispersed sites or campgrounds?
What type of activities are you guys into?
Do you like tourist type stuff or stay away from the crowds?
Another thing you might try to get ideas of places to check out is go to the section for trip planning and downloadable routes and filter by NW region. See where others have gone and if any places peak your interest.