[idek]

A lot of people like a shorter stock on a defensive shotgun. This is because they believe that a defensive shotgun should be mounted with your shoulders squared up rather than angled like they are for hunting or target shooting. They believe starting squared up gives you a greater range of motion and better balance if you need to react quickly.

Of course, a shorter stock means shorter overall length, which some people see as a positive in regard to maneuverability.

The Hogue 12" LOP stock is popular among people who prefer the shorter stocks.

Other common suggestions include a light that, when quickly flashed (which is how they're often taught to be used), helps to verify the location of the threat (and whether it really IS a threat), and can temporarily blind an intruder in some cases, giving you an extra advantage.

A sling is also often recommended, so you can use your hands for other tasks--calling the police, carrying kids to a safe room, readying a side arm, etc.--without dropping the gun.

Some people like a big bright front bead for quick aiming in the dark.

And a side saddle makes sense as well.

Having said all that, the gun I would use for HD is not decked out at all, but those additions do at least seem logical depending on a person's circumstances.

As for additions to avoid, I'd shy away from pistol grip only stocks, vertical/pistol grip forends, any type of optics, or heat shields.