My idea:
Pros are:

+20 fire rate
Makes spies bleed for 3 seconds, when disguised it's 6 seconds and when cloaked it's 8 seconds
Has 2 modes: Double Barrel, and Normal Shotgun. Costs 25 metal to change between each
DB mode:
-50 clip size
-10 fire rate
+50 damage
+10 bullets per shell
Shotgun mode:
-10 bullets per shell

Cons:

-25 clip size
Marked-For-Death if a spy is shot with it for as many time as the spy is bleeding
+10 damage vulnerability from sappers to buildings
Sappers take one more swing to remove

Post your ideas down below!