I think stripes look really nice, especially if used as binding. If you had a diagonal stripe in red and white, for example, it would look like peppermint candy canes around the edge of Christmas placemats.
I would think you have to fussy cut the plaids so they look balanced, but plaids would look good too. Very homey.
I used to buy plaid homespun and make dish towels for my mother and grandmothers. I appliqued crochet shapes and ruffled edging to make them fancy. I don't see why you couldn't make placemats that way too.