I'm just playing with ideas right now. And yeah, the idea for his main is basically a cross between Righteous Bison, a Sentry Turret, and the Brass Beast. To sum up OJ's idea:
Unlimited ammo (via natural ammo regen), small team-colored chain lightning effect/splash, sentry shutdown ability when locked (maybe make the shotgun a targeting array of some sort?), EXTREMELY slow movement speed (perhaps make it so you can deploy/undeploy the weapon by switching to melee - you can move faster, but you sacrifice the ability to shoot immediately), normal spin-up (charge) time, stationary when firing, shot is supa-high visibility team-color lightning bolt effect.
What would be most fun with crits, IMO: Crits vs. non-heavy classes have a chance to disintegrate; crits vs. heavies boost that heavies damage output by 175%/250%/325%, max "chain" length is 4, affects friend or foe.
So basically, when you got the drop, you'd actually get a free-to-open crate, which would give you Tesla's Traveling Turret, Tesla Targeting Array, and Tesla Quick-Transport Tool - main, secondary, and melee slots, respectively.
Okay, I need to get working on this like, now.
TF2-style list of +/-:
TESLA'S TRAVELING TURRET
Unlimited ammo
Crits have x% chance to disintegrate vs. Non-Heavy
Crits give proportional boost to damage output of targeted Heavy
15% Splash Damage
20-second stun vs. Buildables
Speed reduced to 20% when deployed*
Cannot move when firing
*IE, you move as fast as a Heavy spinning the Brass Beast when NOT firing.