Very pretty! I might do a straight line 'ray' design, starting at the upper red corner and radiating down diagonally to the black corner, then one on either side of it, starting from the same point and ending half way up the sides. You could use the serpentine stitch on your sewing machine to do something fancier than straight line. It works with a walking foot. I might do straight line diamonds in the sashing. Again fairly simple but it would give some texture.