Rear bindings either come in the format of a boot or a flip-flop/sandal.
For me, a boot is too restrictive and a flip-flop/sandal is too loose and unsupportive.
I realize there are choices like a radar Hybrid, Reflex rear, etc. but the heels are so cut down that your heel slips out.
Why not make a rear binding in the format of a running shoe? This would support your foot/heel and free up your ankle.