If you like feathers, those wide diagonals of white fabric would be fun to have a long undulating feather on. If you want to keep it simple, diagonal lines (parallel with the fabric stripes in the triangles) would look nice, and could be done via rulers.
If you'd like to play with a variety of textures, Angela Walters recently put out four videos for her
Flora and Foliage Free-motion Challenge Quilt Along, and you could put some or all of those textures in different parts of your quilt. That could be a lot of fun as well as a learning experience.