These would be small turret slot devices with equivalents of EVE-type EWAR functions. The tradeoff is that except for ADS, the driver can't operate the movement of their vehicle and the turret at the same time. A tank driver might switch to a small turret, but the vehicle would be without a driver at that point.

In theory, I'd like proper function of the turret to require skill investment by the turret user as well as opposed to just the person who owns the tank.

Not all EWAR types fit well with the systems we have in Dust, but with some adjustment we could get a basic four types out:

Stasis Webifier
A stasis web turret would be a "relatively" short range turret that would apply a temporary slowing effect in pulses. The other option would be to "slow" based on a constant beam, but it seems from a latency standpoint - a debuff requiring a certain distance and periodic "refreshing" would make more sense. I'm not certain if there should be some sort of "resistance" module to this effect directly, or if boosters/speed modules would just be a natural counter. Tiers of the turrret can increase the range, snare effect, or both. [proposed for Minmatar]

Energy Vampire/Neutralizer
There are two possible functions/approaches here. Vehicles in Dust don't have "capacitors" per se, so the analogy isn't 1:1. However, I'll attempt to describe both scenarios. In both cases, the impacted is modules that have some cycling (duration/recharge):
--Energy Vamp: A Vampire turret, once in effect, would do two things: For the user, it will increase the duration of any currently cycling module, and reduce the recharge rate of any currently recharging module. For the target, the opposite will happen: their module duration is reduced, their recharge is increased. Given the "vampire" effect, the user only gets the "bonus" if the target has modules of that type. An enemy vehicle with only passive modules would be unaffected. This effect serves as balance of sorts to the strength of active modules.
--Energy Neutralizer: A neutralizing turret would have the same negative impact on the target as the "Vampire" turret, but would provide no buff or bonus to the user.
----In both cases, it's a similar pulse effect, and in the Vampire case the user would have a buff for the same period of time that the target had a debuff. Tiers/skills could impact turret range and degree of effect. Both types of turrets aren't necessary here. [in EVE, this would likely be an Amarr skill...]

Tracking Disruption
A tracking disruption turret would have the similar pulse effect of other turrets. In this case, the functionality is to reduce the turret turn speed and effective range of an opponents' turrets. A tracking disrupted vehicle would have a harder time turning its guns to bear on a target, or might suddenly find their shots unable to register on the distant allies of the person doing the disrupting. Damage is unaffected here. There's some possible utility herein for some kind of module that increases tracking to compensate, or a module that provides resistance therein. The pulse effect, like other EWAR, requires refreshing. So, after a time gap, the vehicle would be in the clear functionally. Tiers here can increase the degree of the effect, or increase the range of the turret. [would be Amarr in EVE...]

Sensor Dampening
Sensor Dampeners would be a temporary reduction in effective range/coverage of a vehicles active/passive sensor system. This means, no color change on reticle. No display of the name of target. No reds/blues on Tacnet. There's two ways to do it with respect to limiting the range. You could either "reduce" their effective range (targeting/names/dots still show up in close proximity to vehicle), or turn it off completely. In either case, like the other Ewar, it would work in pulses. So, a vehicle that escapes/flees would have their sensors back online after a given amount of time. In this case, Tiers/skills can impact: the degree of reduction(if it's isn't total), length of sensor hit, and range of turret. [proposed for Gallente]

In all cases, the small turret focus is to increase the need/desire for an actual crewed vehicle. The desire to require skilling by the turret user instead of just the driver/vehicle owner is also an effort to make EWAR something that requires focus and effort to get mileage out of.

An obvious observation here is that I didn't assign a Caldari EWAR, and was unclear on the Amarr. Not all EWAR in EVE can directly apply here because there is no target-locking in Dust outside of Swarms, etc. Capacitor Warfare and Tracking Disruption are both tradtionally Amarr EWAR staples, but one might need to be passed to Caldari balance-wise if no one can help me think of something more appropriate for them.

In any event, feedback is welcome. Thank you!

EDIT: (added a Caldari idea)

Jamming
A jamming EWAR turret would scramble FoF distinction on radar and target for the length of effect. Reds might turn blue, blues might turn red... or they might not. It's also possible that the names could get scrambled. However, if you still knew that X players is always a heavy, and you saw his name on a scout suit... you'd know it wasn't him. Would you know if it was an ally/enemy though? The idea is to raise chaos with your ability to determine enemy from friend for the length of the effect, but it doesn't otherwise impact your vehicle. For obvious reasons, this is most damaging in games where friendly-fire is enabled. [proposed Caldari]