A). More Ammo is More Better. I like extended magazines that reach to the muzzle. I do not find that they unbalance the weapon in any way. Most quality anti-personnel shotguns come this way now. The new Aridus receiver mounted ammo carriers are a great idea and have changed my mind about "side saddles" and other on bard ammo carriers.
B). Modern furniture. There is no reason other than nostalgia to keep wooden forearms and stocks on the weapon. The new Magpul furniture is excellent and what I currently have on our training Remington 870s. There are those teaching shotgun that do not care for the Magpul line on the shotguns. Only thing I can say there is "follow the money". Magpul shotgun stocks or those wanting a pistol grip, a Mesa Stock Adapter with a Magpul CTR. I find these a better choice than the Urbino type system as you can not only change the length of pull instantly, but you can choose your pistol grip and the color of your stock.
It would be nice if there was a good folding stock for these, but I have not seen one that I would use.
C). Lights. The shotgun is predominantly a CQB weapon and so a light is a good thing to have. In years past that meant a Surefire light unit build into the fore end. But going back to the modern furniture discussion, one can add a mini-tactical light such as a Surefire X300 to the hand guard...or remove it...quite easily. And the new M-Lok system on the forend gives many more options.
D). Slings. If the weapon is to be used outdoors, a sling is important, but again...given the development of the market, I would opt for a removable two point sling. This facilitates going sling-less when the situation calls for it.
E). Sights. Again, a great deal has changed both in technology as well as our learning to use it since 2005. If I was equipping a combat shotgun today...one that I wanted to "optimize" rather than minimize, I would add a red dot system.
That red dot would be organized in a similar fashion to what is seen on modern hand guns, with the prevalence of back up iron sights (either rifle or ghost rings according to preference), etc.
F). Barrel treatments. With the advent of low recoil ammunition, I don't see a need for the various choke systems that became so popular years ago. If its something you want, then by all means add it. Manufacturers learn from the secondary market and many new barrels come from the factory shooting very nice patterns.
My suggestion is to pattern your shotgun at various distances from 5 to 50 yards in 5 yard increments and develop some knowledge about what your barrel can already do. Then go for additional barrel treatments if necessary.
Remember, we want predictable uniform patterns increasing in size, not necessarily tight and small patterns.
The shotgun can be a fine weapon and in the hands of a skilled man can do amazing things. But it functions in its special niche and not in competition with other weapons in their own niche. Making sure the fight happens in the area your weapon will have the advantage...well, that is what tactics are all about.