I personally would treat the open spaces as if it was a whole cloth and utilize those to do some fancy, but not "heavy" quilting. Something more than cross-hatch, but less than an entire whole cloth. It looks like the smaller flowers are set in a circular pattern around the large center flowers ... so perhaps a large feathered wreath that covers a lot of the space in between. Outline all the florals, then fill in with cross-hatch.