last i had heard, shotguns are a medium weapon, main for a medium / secondary for a heavy only
which, to me only leaves a few pairings that make any sense
Medium - Shotgun/Short rifle (SMG makes little sense as they are both CQC)
Heavy - Auto-Shotgun/Assault Rifle (easy choice, short and long range)
Heavy - Shotgun/Heavy Weapon (preferably something that compliments the shotgun, such as the automated grenade launcher, you wouldn't want to be firing nades in CQC)
Having a pump action as a light weapon would be nice, allowing many complimentary pairings (such as with the single shot nade launcher), but I haven't heard anything about it. Would even be able to have semi-auto as a medium weapon and full auto as a heavy.