Would any of the various Clethras be big enough? There are several species, and cultivars thereof, in the Cornus tribe (other than C. recemosa) that one or more should work. Smoke bush might do in one of the sunnier spots - there are several cultivars with varying leaf colors. Witch hazels are another family that has several cultivars. Is Fothergilla tall enough? Or what about one of the viburnums - most take shade, and there are varying heights among all the species and cultivars?

If hardy, oakleaf hydrangeas might work - there are several cultivars. Any of the various hollies might also work, albeit they might need the sunnier areas. Any of the various Rhodies, both evergreen and deciduous, could also be a possibility.

My goodness, you have a wealth of choices. Do you think you could give people a better idea of the density of shade? Dappled, light, part shade, deep? What currently grows there...the plants that are there will give us an idea of which shrubs or understory trees will thrive. You have a great advantage in that the soil is deep. If you had norway maples with rooty dry soil, that would be a big challenge. You'll have fun with this!

Here are some ideas: calycanthus floridus, hamamelis virginiana, lindera benzoin and symphoricarpus albus. The symph may not want shade - need to check that. The others do fine under trees, but will likely have a more open habit than if they got more sun.

Here is a link that might be useful: symphoricarpus