I was going to suggest a 1/4" echo inside the blue pinwheels and a square in the centre of the red squares but I like the spiral idea from key4unc better! The border could be stipples or just parallel diagonal lines radiating out from the centre to each corner. I love it BTW - it's such striking colours and a strong design element complemented by your fabric choices. My preference is always to accentuate a strong design like this rather than do an all over quilting pattern which might detract from it. Turning it through the machine won't be easy, I know and my quilting suggestion is coloured by the fact I would hand quilt! But however you do it, it will be stunning and your GS will love it!