I don't know what your skill level is, or what equipment you will be using. But since it is Christmas, and you have snowflakes on the left, some type of snowflake or six-point star would be fun. You could use a vertical straight line as a "trail" and place your flakes/stars at random heights. If you want to go simpler, I would quilt in straight diagonal lines, following your piece lines from upper left to lower right, creating diagonal channels.
Your placemats are very pretty and elegant. I hope you find the perfect quilting solution for them.
Darren