I'd say that for simplicity's sake they should be bound to the same rankings as players - combat, trading and exploration. Everything in the game is one of those three pillars, and it would be quite fitting for NPC wingmen/crew/pilots to also follow the same system.
Combat would obviously involve all combat-related activities, ranging from manning turrets on our larger ships all the way through to piloting their own independent ship.
Trading would involve them knowing what and when to buy for when they help you on trade runs, they could even pick up missions based on their own rank and the destination to further supplement their income and provide you with extra materials. When functioning as crew in your own ship, they would both be capable of automatic cargo management (so you could allocate part of your ship's cargo capacity for them to sort out with their own missions and trade goods to either double up on missions and rare goods or to simply reduce workload on the player) as well as attempting contact with special wares traders or mission gives in space during the journey. When in their own ship, they would simply pick up relevant goods and missions for the projected next destination before flying off in convoy with you. Trading skill would also determine what goods they would bother with while mining, penniless traders would likely just pick up everything while elite traders would only ever pick up the best things for missions and based on local markets.
Exploration would involve a lot of the tech side of things and scanning stuff. Explorer crew members would be able to keep the ship running and masterfully manage local ship systems, while also helping scan everything than can get their hands on (both stellar objects and ships, including relevant cargo scans and wake scans). They could even add a scouting SLF, designed to be launched in supercruise (effectively an SLF that trades weapons and a PD for a tiny FSD), which explorer crew could take out to help scan systems even faster. Explorers in their own ships would be able to help the player scan systems in supercuise, as well as the explorer skill helping with prospecting while mining.
Obviously, most activities realistically involve a combination of the above different skills, which would make crew management a delicate balance. Training a complete A-team of crew would be a massive drain on player income but be convenient, while keeping specialists on call would be a pain to manage but would be better for the bank balance in the long run (assuming FD add in some kind of reduced pay for inactive crew members).Either way, they would need to have a serious review of wing and crew payment mechanics if NPC wings were to become a thing, as currently wing mechanics seem to magically multiply all sources of income in some way, while currently NPCs are a serious drain on overall income. Some kind of balance would have to be worked out, balancing income between pilots of a wide variety of different ranks as well as balancing rewards between those who own the ships in question and those that are having them provided to them.
+1 OP
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