I very much want to see NPC wingmen. With regard to balance the first shift was the introduction of Wings of players only. NPC wings would only help shift that back to a lone wolf player, and like others have mentioned, don't allow mixed NPC & Player wingmen. With regard to things like instancing in supercruise, they don't really need to be there anyway, they could be tied to your ship, and appear with you when you drop from supercruise. Granted it would be better to have them in supercruise but if it's not doable, to me it's not a really big deal, as long as it just shows that you're in a wing when you are targeted (it already happens like this with NPC's quite a bit anyway).
I think the better option is to use crew with OUR ships, that way the risk is greater, as we are the ones who pay should the ship be destroyed, if they had their own ships already there is very little risk. If that risk doesn't seem high enough, there could be things like NPC's aren't covered by insurance, so there is an excess amounting +x% of the ship value to be able to re-buy the ship (and it could also be supplied at the last docked station so you now have to either go back or transfer the ship). This could end up being very costly if you're wing is decimated along with yourself depending on what ships you let them fly but it also enables an element of control via decisions as opposed to hire NPC with ship already determined via RNG and risk is whatever cost it was because if they die, they die so what, it was just some stock ship and generic single use NPC anyway. Also a side thought, if it was hired crew, and they do get the ship destroyed, unlike if they were regular crew, they would have an escape pod, but you'd better scoop it up before you leave if you want them to survive.
NPC wings also allows for more challenging scenarios, FD could scale some encounters based on numbers in the wing, so it could actually be safer to be alone sometimes than in a wing. Police might take longer to arrive if you're in a wing and are attacked, and like others have said they might just not be willing to attack other clean players, and naturally you would be responsible for any bounties & fines they accrue while under your employ, and they accrue them the same as you do, hence a single 400 assault becomes 800 (but still split so time is same as 400), which upon death, are contractually passed on to you (with relevant time increases based on new amount). Should you have an NPC wingman and you accrue a Pilot Federation bounty (assuming they become a thing and are only awarded for clean player destruction) then the wingman might simply desert you (with your ship, as ransom till you pay there fines so they are clean again, or perhaps with your ship as payment) until that time in a non-consensual PvP, they could hang around not firing but still be able to launch limpets (hatch breakers, collecters), scoop cargo do scans etc. Meaning they would be liability in a non-consensual PvP encounter where you are the instigator and your goal is to destroy and not rob someone. Attacking clean NPC's could be different as there wouldn't be a risk of getting a Pilot Federation bounty so they could attack (again if PF Bounties become a thing but I'm simply saying it this way to illustrate that NPC's aren't meant to be used for non-consensual PvP fighting, but could be used as support for pirating, and used for defending against wanted commanders).
If it was consensual PvP, they couldn't be used with this rule-set but why would consensual PvPr's even want to use them ? (it could be tweaked to give them god-like knowing abilities of if report crimes is on or off, or that setting could be made known to players avoiding the god-like knowing powers part, but it could be done if there was actually a demand for it). But in essences they won't fire weapons if they risk reducing their employers proposed "karma", which should always be on a Player not on an NPC).
If a system like this is used I don't think NPC wingmen would really need to be limited to certain ships, as they theoretically couldn't be used as offensive firepower in a PvP encounter, only as defensive firepower. PvP oriented players would just have to use Player wings instead like they already have been and do (but I'm sure some will still think it's suddenly more unfair somehow). I think the biggest issue with NPC wings with the current set up is the orders aspect, and it already exists with SLF's i.e. can't get them to do scans, launch limpets, that kind of thing, but nothing that couldn't be fixed.
