I'd like to see guns that can be used to great effect when moving around (with good accuracy) but weak enough that in a straight up firefight, they lose their effectiveness, either by not having enough ammo to get kills, or too wide a spread to work effectively at close range. The shotgun is not such an example at the moment, as its recoil is greater than the rifle's.
On the flipside, I'd also like to see weapons that are exclusively good at suppressing. Perhaps they could protect the person manning them by giving a narrow field of vision (frontal armor) but rewarding the suppressor by allowing them to spew bullets in front of them at their leisure with a large amount of ammo.