First wish:
Rifle scabbard like the Mountain Rambler but removeable. It would attach to the top of the frame and ride inside the bag itself. Give the option when you order a bag to have a Velcro opening that the scabbard will slip through just above the bottom of the pack. Put clips on the Velcro opening that attach to the frame so it doesn’t compromise bag strength.
Maybe not exactly what you are looking for, but close?
Kifaru Long Range Hunter Scabbard with Lid - The Extreme Store
Kifaru Long Range Hunter Pack - Full System - The Extreme Store