Everyone likes to have accessories and modify their shotgun in one way or another. Some people will never modify or upgrade the gun, but those same people will buy a precisely fit sling or unique protective case. Those that do upgrade will add any number of add-ons available depending on how they plan to use it. When thinking or accessories or upgrades the best thing to do is to look at all of the products made available for the most popular shotgun in the world, the Remington 870, and then see if that same item is made for the shotgun you own.
The reason you look to the 870 is because the 870 has more accessories and upgrades available than any other shotgun. This way you are not limited to imagining items available for your specific shotgun, you are seeing the entire breadth of available products and then if it is not made for your shotgun, you can have it made.
Some of the accessories available for any gun include shell holders, slings, cases, and cleaning kits. They also include special storage and safety locks. Upgrades include everything from magazine extensions through adding a full length tactical rail. For example, if you are making your 870 into a tactical shotgun, there are hundreds of Remington 870 tactical accessories and upgrades available. Muzzle brakes, pistol grips, other stock configurations, attached shell holders, and even red or green dot sights. You can even upgrade an 870 to do everything you would expect from a Saiga 12 drum combat shotgun, save for the semi automatic action, if that is your goal and at the same time enjoy the widespread availability and affordability of parts. This is the same for any shotgun, not just the 870.
If combat or tactical use is not your goal then while looking at the options for the 870, you can see that any upgrade or option imaginable has already been thought of and put to use. Different barrel lengths, replacement fore-ends, heat shields, unique stocks, and mounted flashlights are common upgrades that make any shotgun unique.
The important thing to remember is that you can get accessories or upgrades all shotguns. Some will have to be custom made or ordered if you do not own an 870, but looking at what is available for the 870 opens your mind to the possibilities