If you don't want to quilt over the colored quarter circles, I would definitely stitch in the ditch along the black and white sashings which go through the circles. Then, if you look at the space between four circles, the space where two white corners and two black corners meet, it is roughly a square shape with curved sides, kind of like the shape of an improved nine patch piece. Maybe you could find a "block" quilting pattern you like to fit into those areas. It would be nice if it were an Asian type of pattern, I think.