Folding barrel would be interesting.
I had considered something on those line back when i had access to a machine shop but decided it was beyond my skill to pull off.
I was considering a compound hinge for the barrel and setting up the cocking link run like a T-slot over a stud so you could disengage the piston with a push button on the side of the stock thereby allowing you to fold the barrel right back to the trigger guard. when the link was engaged the travel would be self limited by the piston travel and work like a regular break barrel.