Certainly no expert, but it does seem a good spot for a spicebush, Lindera benzoin, or, maybe, a large clumping fern, like Goldie's. I have some evergreen woodferns that stay in one spot and grow about two feet tall. I do not know how they would do in your zone.

Interesting ideas. I just looked up Lindera benzoin and although that would be great, it said it will get 10' wide and high which is too big for this spot.

Hmmm... anything else?

Astilbes are my favorite shade plant and some of them can get fairly tall. Newer varieties also bloom longer but I don't know any names. Foxglove may also work although it is a bi-annual. I am going to start seeds soon so I will have flowers next year. I cut the large bloom stalks from several of my foxglove once the flowers started to fade and they have put out several smaller bloom stalks with smaller but very sweet flowers. I also have Pacific Giant delphiniums in a shady spot that are very impressive and have just finished blooming. This is another plant that will put up more bloom stalks if you cut them back. I hope you will show us what you decide on. I love seeing others gardens.

Bonny

How about Kirengeshoma palmata (or the lesser species koreana)- Yellow Waxbells? It fits your situation and measurement requirements. They have nodding yellow bell-like flowers and large, Maple-like foliage. But if you wanted something more tall and upright you might want to go with Ligularia or Actaea. Hope this helps.
CMK

May I suggest:

monkshood
goat's beard
joy-pye weed
queen-of-the-prairie
meadow rue
Ligularia

Libby

Thalictrum and astilbe would be my choices for a moist, shady location. I know you said a single plant, but I really like the look of 2 or 3 slightly varied shades of astilbe grown together.

I was thinking of astilbe or goatsbeard. I have coral bells, painter's pallet, peonies and some lower growing plants in and around my lily of the valley, but it's a little larger area.
Sue

Thanks so much for everyone's input! I am going to do a little research on all of your suggestions and hopefully I can get some things planted soon.

Thanks again

Just thought I'd throw this in -- some friends of mine named their daughter 'Lindera' after the shrub mentioned early in this thread. The father just loved it.