Were you thinking of replacing the art work aleady there, or having something above them? I think I would probably approach the project by deciding how big I wanted the finished piece. Maybe taping a piece of brown paper above the bed with painter's tape to make sure the size is just right. Then from there, think about orientation, do you want borders or do you want blocks going right to the edges, do you want something with lots of detail and small pieces, or do you want something with a more dramatic central design like a medallion style? I could see going a lot of different directions. A large floral applique with a pieced border would fit the traditional look nicely. Or you could do a bargello with sublte color gradations much like the bedspread, or you could do pieced blocks such as stars. Really, the possibilities are endless, even within a "traditional" framework.