OH OH OH I just love this. You know, do you struggle with stippling?? I think swirls in white thread in the white areas and something that would blend in the darker areas would look FANTASTIC on this project...all that white space is just begging for texture!!!
Swirly circles wouldn't be too difficult if you feel confident with stippling alredy - just practice drawing on a sheet of paper a swirling circles design without lifting your pen...then try it with thread!
Just a tip - you are going to drive yourself batty with all those pins! Every three or four inches should be plenty sufficient to keep you from sewing wrinkles in!
(It looks like a smaller quilt from the picture - if it's larger you're right to use lots of pins!)