I like the idea of including arrows going down to balance the ones you have going up. I've seen several pinwheel patterns. Without pictures, I'm having trouble visualizing it. Just saw a quilt show on tv where one of the tips was to print out colored pictures of what you wanted to think of using. Print to the size you would need. Cut and paste the paper images of the fabric and try the pattern out. Seems like it would be less cost and time that cutting and sewing fabric samples.
Can't wait to see pictures of what you decide on.