I think I would leave the cornerstone out. The borders make a bold stayment and I don't think it needs anything else. BTW this is a very nice quilt. Did you use shirts in this or is that fabric that looks like shirting? Love how you used the tan sashing with the red cornerstones in the body of the quilt, brings each block out, so the eye lingers on one at a time.

I would leave out the corner stone and bing the quilt in the red and it will balance out your quilt and border.