a light weight (single sided) flannel is nice- adds substance without alot of weight-
also good quality muslin will surprise you! muslin washes up to be one of the softest most (cozy) cottons out there...and works well for summer quilts.
when quilting the density required depends on the properties of the batt being used- so if using fabric as a batt you can quilt any way you want - no density requirements because it is woven and not going to come apart like batts pull apart.
if going with flannel there are single and double sided flannels- pick a single sided one for the lightest weight.