I did a wavy cross-hatch on placemats years back. I had only made 1 quilted table runner, SITD from a kit prior to that in my quilting "career", but they turned out lovely. I was able to do the gentle waves without a darning foot (didn't realize I needed one ... or how amazingly fun & easy free-motion quilting actually is). I was really happy with how they turned out. Wish I still had them because I really can't draw (I bought a borders template at JAF & used it to mark my quilt), but here's a really rough idea...
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You could also just quilt the diagonal lines going one direction & that would look nice, too.