It's true you can go too far, but the right add-ons can greatly increase a scattergun's capabilities.
Some people go overboard, hooking things to every free Picatinny rail spot they can find. While there are a ton of aftermarket accessories for popular and affordable shotgun models, like the Mossberg 500 and the Remington 870, which allows people to spend a bit on a gun and a bit on some upgrades instead of a lot on a Benelli or Beretta.
Some shotgun accessories are extremely useful and greatly expand the applications of most any shotgun. Anyone can go too far, but if you keep a solid equilibrium of weight, bulk, and practicality, you'll be alright.
First, let's talk about the buttstock. Plain Jane 500s or 870s on the lower end of the price range come with a straight, traditional polymer shotgun stock. Some people prefer something that a little more customizable, which will make the gun more comfortable to shoot.
A common an inexpensive stock option is a pistol grip with a military AR style telescoping six-position buttstock, which can fit most lengths of pull. Some people don't like a pistol grip on a shotgun, feeling that it transfers too much recoil to their hand. Fortunately, there are alternative stocks that are still quite adjustable, like those offered by Magpul. These stocks will often come with multiple sling swivel attachment options, from QD cups to old school sling swivel studs. They will also typically include significant recoil pads, or recoil pads of any thickness or absorption capability can be added later.
Ammo: Capacity and Extra Shells
A tactical shotgun, to be a tactical shotgun, has to at least have an increased ammunition capacity over a hunting model, usually achieved by an extended magazine tube that's typically the same length as the barrel.
Even if a shotgun has an increased capacity, it's a good idea to keep a few spare rounds on board the gun itself. Some buttstocks have an optional shell carrier that can be mounted on the side opposite the shooter's cheek. A sidesaddle ammo carrier is also a popular way to carry extra shotgun shells. They are typically polymer and mount to the receiver on the left side and hold six shotshells.
There are also many slings with shell holders sizes for 12-gauge or 20-gauge shells. Some are one piece with a top rail. Keep in mind, this all adds weight to a shotgun, which can already be quite heavy.
Gun Lights and Lasers
Even a double-barrel shotgun can benefit from a mounted gun light.
For low light situations, identifying your target is of paramount importance. Some shotgun forends will come with accessory rail segments or other ways of mounting a flashlight. These days, many include M-Lok rail system slots instead of rails, so the user can add just the rails they need.
If that's not an option, there are various accessories that clamp to the barrel or mag tube that allow you to mount a flashlight.
A lot of people elect to mount laser sights on their shotguns, for the simple fact that they work really well, and these days, laser sights are really small and light, so there's not much of a disadvantage if you have the rail space. The Crimson Trace Lasersaddle takes up no rail space while adding a top accessory rail and allows shotguns to be fired accurately from the hip, especially guns without stocks like the Mossberg Shockwave.
Optics and Choke Tubes
Many shotguns come with receivers already drilled and tapped for rail segments for the easy mounting of popular red-dot sights and other optics, which are excellent for close range shooting and fast target acquisition--great benefits on a 3-gun shotgun. Shotguns are quite forgiving when it comes to optics. If it can handle the recoil of the shotgun and hold zero, it should work just fine.
If your shotgun has a barrel threaded for them, it's a good idea to get a set of choke tubes. These will allow you to fine tune your shotgun patterns at various distances with various types of ammunition, and make the gun more versatile overall, as it can be more easily used for hunting or something competitive like skeet shooting.