You're so smart to leave an 'empty' space for your design wall. I hope you'll get plenty of suggestions for how to make one.
I made a sleeve on the top of a big piece of white flannel. DH put big cup hooks on the wall to hold the dowel we put through the sleeve. When I made my on-point watercolor quilt (the one that I lost), I marked 2" squares diagonally on a white piece of flannel for a sleeve. That one was done when Mom and I shared a sewing room in her BIG apartment in AZ. Of course, now we all know that the EASIEST way to do watercolor quilts is with the pre-marked adhesive backing. But flannel still makes a good design background. I've heard that the back of a Vinyl tablecloth is EVEN BETTER, but I haven't tried that yet.