I'll start this off:
Accessories: Night vision scope, though unrealistic, would be a great idea for guns like the longshot or recon, or even a REAL laser target. What's with the big red blob on the tactical laser beam?
Features on guns: A tactical rail on the side of the gun, for attaching the scope so that you don't have to lean over the gun to aim, just have it there, compatible with shoulder stock and everything.
New guns altogether: one that has 2 barrels, like the barrel break, but with 2 clips that would fit underneath, giving less reload time and a higher dart capacity, though it ma have an impact on the power of the gun.