Soldier: Possibly the ability to deploy an ammo crate that teammates can get ammo from for a limited time.
Medic: A passive ability much like the universal ability Resupply Rate Increase, that would further increase the rate at which your supply goes up. (Because even with all the supply passive abilities, it's very easy to run out)
Engineer: Possibly a deployable short wall for cover to shoot from behind, of course if you shot at it enough, it would brake.
Operative: A buff to increase their own or their teammates running speed until they die. (I know medics have a buff like this, but i think Op should have it instead, because they are more speed based and do not have any buff.)