I don't like stitch in the ditch because if i am going to quilt, i want to see the quilting and you can't see it w/ stitch in the ditch. Just my opinion.
For the outside/borders, I would do outline quilting - coming in 1/8th to 1/4 inch.
I don't have any ideas for the center - glad you do!
And yes - it is a beautiful quilt.