This suggestion is based off of Day of Infamy's game mechanic, where every class has its own small, but slightly useful ability.
Aside from the Commander, since calling in air support is already an ability, the rest of the classes in INS:S could have their own too.
Before the mass deletion of BETA1 forum posts, I had a post that suggested that the Observer class should have the ability to capture objectives slightly faster than the rest of the classes.
So here are my personal opinions on what the rest of the classes should have for an ability (Assume that these abilities are small increases of certain gameplay mechanics:
Commander: Call In Support (Already In-Game)
Observer: Faster Capture Rate
Rifleman: Faster General Speed
Breacher: Faster Breaching
Gunner: More Ammo
Demolitionist: Stronger Explosive Defense
Advisor: Faster Switch Between Weapon & Equipment
Marksman: Longer Hold Breath
If you guys have some ideas, I'd like to read them!