Shotguns have character, so here are some ideas for unique shotguns. Enjoy.
Serious ideas:
Furious Blast - combat shotgun.
Shells fired from Furious Blast explode. Small radius, but several explosions per round would ensue. It could be used to blow up walls and doors, and of course pulverise enemies.
Igniter - double barrelled shotgun.
This shotgun is so powerful that the gunpowder explosion within the shells that issue from it produces a cone of fire from the barrels, burning any creature that is one square from the player in the direction it is shot, rendering upon them damage roughly comparable with an Arch-Vile's attack.
Skull Flayer - shotgun.
This shotgun fires a single projectile, rather than many. But what it lacks in quantity it makes up for in raw damage that isn't reduced by distance, rotating and expanding upon impact to tear through flesh and leave the enemy broken. The shot will go through several enemies, coming to a halt only upon hitting a wall or door.
Speculation:
Beckoner - double barrelled shotgun.
This acts like a normal shotgun, unless you 'R'eload it. Then it uses its Harpoon attachment, which uses 2 shells to blast a barbed harpoon at a single enemy, sticking into its flesh and pulling it, kicking and screaming, a few squares closes to the player with a chain.
The harpoon attachment is mounted in one of the barrels, and the chain it trails is welded to the gun with a hydraulic winch. Depending on the size of the enemy and the player level, the gun may be whisked out of the player's hands rather than pull the monster.
Impaler - shotgun.
This brutal shotgun carries a long pointed blade above its barrel, which can be used to make melee attacks. Once a melee attack has been successful, and the enemy impaled upon the spike, the player can fire the shotgun, which, at such close range, will do terrible damage, such that it overcomes armour, cannot miss, and fires all its projectiles into that one unlucky monster. If either the player or the enemy moves, the Impaler will come loose from the beast and another melee attack will be required.
Collector Deluxe - shotgun
This is not so much a shotgun as it is a device unto which shotguns can be attached. There are mounts for up to seven guns to be inserted, be they shotguns, combat shotguns, or double barrelled shotguns. Each gun needs to be loaded seperately (with the R button), similarly to the combat shotgun's partial reload. Once all the shotguns are set and loaded, the wielder must turn a crank handle until the mechanism clicks into place. That done, a pull of the device's trigger will fire off all the attached guns at once, causing all manner of havoc.