Quilters Dream Request cotton is one of the lowest loft battings on the market even thinner than W&N. Silk is also very flat but you will pay dearly for it. Some of the bamboo ones are pretty low loft as well. Also you can lay a lot of thread down on silk or wool and it won't get stiff like W&N does. So you can go with very densely quilted to get it flat and still have nice drape if you go with wool or silk.
You could always just put a layer of flannel in as batting but your quilting will have pretty much no definition.