gorgeous quilt! I've never used flannel in my quilts, so I won't comment on that. If you do use muslin, I suggest a premium muslin (200 thread count). You can find them on sale about 1/3-1/2 the going rate of prints/batiks at a LQS. Kaufman, Southern Belle, Legacy are all very nice muslins. You can get wide widths also if you don't want to piece the back. I like the idea of a light green. that would be really pretty. a marble/tonal/small print would complement your top nicely.