If you choose to leave it as it is - which I think you should - because it is adequate as it is for a blanket/quilt -
how are you planning to anchor the layers together?
If you did not shrink your flannel before sewing it, I suggest "tying" it -
I just saw a video on youtube describing the "hidden tie" or "international stitch" - which I think would work well on this quilt.
https://www.youtube.com/watch?v=cEkHht2oJsg