First, let me say your quilt is beautiful. I would pick a line(ie top left of large and small blocks) and do a simple straight stitch, going in both directions, creating diamonds(cross hatching). Your quilt is already geometric, and the rule of thumb is for quilting curves for geometrics ( I think), but your quilt is already beautiful, I think the simpler the quilting, the better. JMHO! Another idea popped to mind as I was proof reading what I had wrote... curved cross hatching.