I like HC's idea.
"Automatics" as a skill. Could be broken into a few different classes across the spectrum of progression.
Machine pistols that are relatively high damage for a 'lower' tier weapon but have terrible accuracy. No fire select, for balancing reasons. Inspiration being things like the Glock 18C, Skorpion, HK VP70.
Subguns as they are more or less now. It'd be nice to see some commonly-used modifications for these added. Sorely missing are foregrips, as most sub guns have relatively poor or no heat shielding and barrels that get insanely hot. It'd be great to see a bit of modernity/futuristic styling on these, perhaps including things like the MP-7, HK UMP-45, Kriss Vector.
At the high end, parallel options for fully-automatic rifles. Given the surge in popularity of pistol-caliber AR pattern rifles in recent years, I can only see these as logically being wildly popular in the legal constraints of Withmore. All the accuracy and stability of a rifle platform. Examples would be things like the SIG MPX, Colt AR-pattern 9mm rifles (these could be refit for a higher caliber for balance reasons, such as 11mm.)
On the modding front, smudges could surely benefit from foregrips, stock buffer pads, laser aiming modules (I think these already exist? Maybe just in descriptions, and not function?) and holographic sights. Primary consideration for lightweight automatics is generally target acquisition and recoil management.
For rifles, I think there could be similar parity here.
Shotguns could have their own progression. They need a bit more representation at ''high'' levels. Perhaps something that can take a drum mag at the high-end. Pure ballistics aside, one of the lasting strengths of shotguns on the modern battlefield is their ability to take tactical loads. Something I'd love to see in SD. As mentioned earlier in this thread, Flechette rounds offer superior armor penetration to traditional ball ammo buckshot. As do armor-piercing slugs, which often take the form of fin-stabilized discarding sabot rounds. Explosive rounds and non-lethals are also very much a thing for shotguns. For the sake of game balance, I'd imagine that explosive ammunition could simply be treated as doing blunt force trauma and not like, being totally overpowered.
Mods for shotguns could be chokes for additional range or accuracy, magazine mods for shotguns that don't innately support them, extended mags for ones that do, holographic sights, laser aiming modules, or perhaps some kind of rapid-reload mod that would let you use the weapon without having to spam reloads regularly.
Finally, for rifles, in the mid range, I'd expect to see some carbines and designated marksman's rifles. High-caliber ammunition is very much lacking in this regard. We have our two ''super calibers'' for rifles, but we could easily hand-wave and say that there's semi-automatic pistol-caliber 10,11, and 12mm carbines. Options for this could be things like the long-barreled version of the FN P90X, Steyer AUG which has a marksman's version, Tavor X95, SIX MPX (long-barrel version).
For Mods, I think that scopes should be dropped in price. They're currently priced the same as TruSights (What?) and could probably come down in price. Alternatively, offer a lower-power version that can only zoom a fixed 'distance' at a budget price, and then allow higher power versions to retain their price. For bolt-action rifles, straight-pull bolt modifications are somewhat popular, if machinework intensive mods. This allows the marksman to fire at a much higher rate, since they don't have to re-acquire sight picture to the target between rounds. Accurized floating barrels could be a re-imaging of an existing mod that does similar functions now. I'll admit I don't know pretty much a single thing about loading ammunition in SD, but in the accuracy world, ammunition is literally everything. This could be an additional enhancement for another time, perhaps. Allowing techs to press quality small-batch ammo for the purposes of ''accuracy'' which perhaps could translate into a critical chance bonus in-game.
Then the totally obvious thing everyone in the SD gun world wants: Some kind of superglue solvent we can burn flash on to remove those epoxied-on mods with! This could open up a whole secondary economic market for stolen and black market mods.