I guess I quilt a lot heavier. I'd suggest that your 'pebbles' fabric be quilted a bit more. I would make rectangles, about the size of my palm. It wouldn't take a long time, and it would continue the concept. You would make it look like a rock wall. I would not make the rectangles have sharp corners, I'd make them rounded. For quilting motifs, I have a card file that has many different motifs, for free motion, I divide them up into rounded, meanders, feathers, sharp points, etc. I've found that it really improves my quilt if I add various related motifs.
Good luck.