Tree cluster base plant suggestions please!

Hello All, I'm located in Minnesota, my backyard forms a little natural basin (roughly 90'x150') with slopes on three sides. In the lower section stands this beautiful cluster of 80'+ tall cottonwoods. Its pretty much the focal point of the area and thus serves as a starting point for my woodland garden. I would like to plant comfrey (both for its visual impact as well as overall usefullness) around the base of these trees similar to the way many ppl use hostas to trim out the base of trees. My question is, are there any other plants that I could or should intermix that would add extra color, serve as a ground cover around the comfrey (clover or something similar), look nice in general, of be beneficial to this area? I would like it to look full like the hosta pictures below and match the woodland garden feel and I thought it might look bare when I harvest the comfrey leaves from around the trees.
Last question, are there any mid level trees (I really like maples) that I could plant within this cottonwood stand to add color or would the existing roots of these monstrous trees prove to much for that?