Currently gunners in vehicles can do basically two things: spot and fire. That's not much and aside from the opportunity to fire a bigger gun than while on foot it can get very boring. Gunners in vehicles could easily get extra tasks, they have a ton of buttons at their disposal and each button could be allocated to a specific task. Example: shift is used for sprinting and in vehicles it's to use any afterburner fuel to accelerate. Keeping this trend alive a gunner could have an ability that boosts the internals of the tank. This could be anything from repairing the tank to a temporary speedboost or a stronger engine for climbing to higher velocity shells etc. Ofcourse you have more buttons, so you could allow a Gunner to use spotting devices like a directional radar pulse or firing a tracker so you can follow a target for a minute. Theres the option for using debuffs, alternate fire modes, buffs for the vehicle, buffs for nearby allies, passive deployables like a short-term shield generator or being able to relay information much more easily to nearby friendlies and more. To actually give the gunner something to do it would be best if the gunner has access to multiple of these abilities at once, but the energy to use them is shared amongst them all so you need to chose which one you use. Additionally some abilities could consume ammo in your magazine so you get the trade-off of firing another shot or using an ability.