All the unique shotguns (Plasma, Jackhammer and Assault) are variations on the COMBAT shotgun. Here's a couple of double shotgun/regular ideas I've been throwing around:
Shotgun Bayonet
Description: "Do you really want to waste ammo on those chumps?"
Attack time (Ranged): 1.0s
Attack time (Melee): 1.0s
Damage (Ranged): 7d3*3
Damage (Melee): 5d3
Melee accuracy: +3
Spread: Double Shotgun
Reload: 2.0s
Clip: [2/2]
Ammo: Shotgun shells
Special: Melee kills take no time, like butcher's cleaver.
Why?: Fun, mostly. Offers little strategic benefit for the dedicated shotgun user, unless the melee damage is buffed, but that would result in it being better than the chainsaw and we don't want that.
Name Pending (Based off Metal Slug's Shotgun)
Description: "The most effective of the peregrine falcons' famous weaponry. It cuts tanks to ribbons!"
Attack time: 0.2s
Damage: 9d4
Spread: Very short (Shorter than DShotty), reasonably wide. Damage does NOT dissipate over distance.
Reload: 2.0s
Clip: [2/2]
Ammo: Shotgun shells
Why?: Buffs Active Shotgun, as Passive Shotgun is all the rage these days. Could be implemented as an exotic weapon. Also gives shotgun users a "Wasteful but incredibly effective" alternative. Also clearly the best Metal Slug weapon.
The Shotcycler
Description: "Oh yeah, now you're gonna make some demons your bitch."
Attack time: 1.0s
Damage: 8d3
Spread: Normal Shotgun.
Reload: 2.5s, move-reloading only reloads one shell. You MUST load 6 shells at once if not moving.
Clip: [6/6]
Ammo: Shotgun shells
Special Fire: Chainfire. Fires all 6 shells at a rate of 0.5s, allowing you to individually aim each one. Aborting chainfire results in the clip being emptied.
Why?: Why not have a daikatana reference, huh?