Very cute tree skirt! I love the crosshatching in the center. Check out Diane Gaudynski's book Guide To Machine Quilting. She has a technique in the book for freemotion crosshatching using a richochet motion. I'm not that good at freemotion and it was a nice technique. It does take some practice but it eliminates all of the the stopping and starting and gives a nice effect. She also discusses what and where to begin quilting which will help with some of the puckering issues. My local library had a copy. Yours might as well.
Otherwise, I like the idea of diagonal stitching or lines coming out from the center. Either way, it will be beautiful and I look forward to seeing your finished piece!