I'm not a fan of leaving large areas completely unquilted. I think you could get some nice contrast in texture by quilting a different design in the background vs. the leaves, and even a different density of quilting, but not completely unquilted-- to me that just looks incomplete. It's really hard to give you specific suggestions without seeing your wallhanging, but I can envision doing something like a gentle rippling meander (think reflections in a pond) for the background, and then doing a double outline of the leaves just inside the leaf portion and maybe adding some detail -- veins or swirling fronds or curves-- whatever would look nice.
When I'm deliberating on how to quilt something, I often will either photograph or photocopy the quilt and a single block and make several copies. Then using a marker I will doodle until I find a design, entry and exit points, and a flow for stitching order -- saves time with the dreaded ripper.