These are the results of a short brainstorming session. Some of the following ideas would be difficult to implement and balance, but most of them are used by the military today. I tried to keep in mind the kind of time period SD has in mind, a not so distant future where most things are futuristic but still recognizable.
A laser sight to improve recoil recovery, stability, and/or accuracy - the disadvantage is obviously that it would tend to give your position away.
A bit more far fetched idea would be to adapt the Cornershot weapon system for the game, would allow a player to look and shoot around corners from safety, but would increase a pistol to light weapon status. I'm not sure how balanced this would be, but it IS grounded in reality.
Some sort of sensory overload attachment for rifles to cause an effect similar to flashbangs. Could balanced similar to a grenade launcher attachment by giving it a limited number of laser or light "blasts" due to a small battery. The military is currently exploring using powerful lasers to completely disorient anyone who the weapon is being pointed at. The real world application is that while you have your gun aimed at someone, it is nearly impossible for them to react or shoot back, allowing you to handcuff or interrogate.
A laser marker, would allow you to point out just about anything you like on your teams minimap, making it easier to point out enemies, objectives or plans. Could be very useful when combined with simple voice chat. Again, lasers could give away your own position though.
SMG recoil compensator. Adding basically a large chunk of metal or similar weight to an SMG can improve recoil management with the obvious disadvantage that the weapon weighs more.
I'm not sure if the concept can be made into an attachment, but some form of radio or minimap jammer could wreak havoc, especially in a game focusing on teamwork like Brink does.
Some form of ballistic shield to be combined with a pistol to make it into a medium weapon. Would pair very well with squad based tactics.
I would also like to see different scopes that aren't just a clone of another scope with a new skin. This could be accomplished through having scopes with different stat bonuses, such as sacrificing fire rate for accuracy when zoomed in; having abilities like a 3 round burst when zoomed in; or maybe have scopes with different augmentations. Scopes with heat sensors, range-finding, information displays while zoomed, or even some form of digital outline to highlight enemies. Again, the last idea may seem sci-fi, but augmented reality is already being adapted to HUD in high end cars, where the windshield would actually highlight things like animals on the side of the road, alert you to important street signs, or even outline the road in bad weather.