It depends on what effect you are looking for. If you want to just enhance the design, I would probably do a crosshatch straight line motif. If you are looking for texture a meander or loopy meander all over would be nice. I agree with Stichnripper that Angela Walters has some great videos on choosing designs for areas (diamonds, rectangles, circles, ect.) and would be a good way to start. I'm doing a Cluck Cluck Sew pattern that has loopy meander in the background and the print block squares use a line design to follow shape of strips with the center diamonds using an echoed continuous curve and diamonds between blocks have a feather design. Test your thoughts out on paper or better yet a clear vinyl or plastic that you can lay on block and draw on with erasable/washable markers (just make sure not to get on fabric)