make your gun overweight, inaccurate, and unwieldy.
The accessories to make your shotgun look cooler are fine for doing that but not for survival. A pistol grip with no stock is not going to do anything but make your gun impossible to aim not to mention the recoil whereas a full length pistol grip stock will improve your aim. Heat shields don't do much other than add weight and get in the way. A magazine extension might be considered but is one extra shot really going to make all that much difference? Chances are NO!
When you start to look for upgrades remember the phrase K.I.S.S. Keep it simple stupid. The more doodads and majigger things you add the harder its going to be to disassemble and repair. Another thing to remember is not to compromise the shooting ability, accuracy, or reliability of your shotgun. Here's the list of upgrades livetoseetomorrow feels are important.
Flashlight mount or Surefire forend
If the intended use of your shotgun is close quarters home defense you probably do not want a sling. However if you will be bringing your shotgun along in a bug out situation the sling is definitely a worthwhile investment. A heavy shotgun is much lighter if you can carry the weight on your shoulders.
If you are planning on using the shotgun for a survival type situation another option for a sling might be to weave one from paracord. You can never have too much paracord.
Having extra ammunition easily accessible is of the utmost importance in a defensive situation.
Most hunting shotguns come with a 22" or longer barrel. This feature is excellent for hunting but in close quarters not quite as desirable. The law permits barrels as short as 18" which are much better suited for defensive purposes. A shortened barrel will lighten the gun a bit but may also decrease accuracy .In a SHTF situation a hacksaw could be used to shorten a barrel but in everyday life the law says you need a permit for anything shorter than 18".
Over sized safety button
Factory safety button can sometimes be awkward to use or nearly impossible with gloves on. Upgrading to an over sized safety button has several advantages. It's a lot easier to locate a huge button than a small one in low light conditions.
This is an excellent mod, if you plan to shoot for any extended amount of time a recoil pad will save your shoulder. For home defense this is another option you may want to skip.
Most shotguns have decent sights from the factory. Some people upgrade to red dots or ghost rings. Both are excellent mods for hunting but in home defense your target is usually 3-5 yards away so they are impractical to say the least. If you plan to shoot slugs 50-100 yards away this may be an upgrade worth your money, otherwise use the money you would spend on this as a cosmetic upgrade to buy more shells.
The mods you choose for your shotgun can improve it dramatically but only if they are quality upgrades that do not affect your shotguns abilities. One other thing to consider is looking up common problems with the particular model shotgun you have and upgrading the internal parts that are problematic such as the follower or carrier spring. Always keep extra parts in your bug out bag for things that commonly wear out.