As as fellow PNWer, I have the following recommendations/thoughts:
Umbrella Plant (Darmera peltata) This is a very large bold PNW native that grows well in full/deep shade. It gets huge so needs room to spread out. As its common name implies (Dinosaur toes), the roots will 'walk' to increase the plant. I like to use it in the back of a shade garden. It sends up long stalks with pink flowers before the leaves break dormancy. Kind of an exotic looking plant. I love it. Might be too big for your use, but thought I'd throw it out there.
Lady's Mantle (Alchemilla mollis) This is a short airy plant that tends to reseed itself where conditions are right. It grows nicely with ferns, and has a mass of yellow-green delicate flowers that last quite a long time. Eventually they flop, at which time I just cut the flower stalks back and let the foliage be the focus. It has an interesting habit of 'cupping' raindrops at the base of the leaves which is kind of magical looking.
Firetails (Bistorta amplexicaulis) Any of the mountain fleece varieties will stand up well in shade (without reaching for the sun).
False Lily of the Valley (Maianthemum dilatatum) This is another PNW native, makes a very nice lush ground cover, which would go nicely with your ferns. I have this growing wild all over in my back woods and it tends to migrate up to my flower beds and establish itself where conditions are good for it. It does go dormant in the mid-summer heat.
Pacific Bleeding Heart (Dicentra formosa) A small native bleeding heart. Similar to maianthemum, this goes strong in the spring and then dies back in the summer heat. I just pull it when it looks ratty, and it always returns.
If you don't fight with deer, hostas all like shade and go well with ferns. I'm also taken with any of the ligularias but these tend to get eaten by slugs on our side of the slope (they do much better in eastern Washington)
Good luck, post pictures - it's always nice to see what folks are working on!