I think the main characteristic developers should focus on if they choose to develop NPC crew, is frequently updated dialogue and evolving personalities.
There's not a whole lot you'd want your NPC crew to do on the bridge, except maybe have a navigator/co-pilot who accepts commands like Plot a course to [System Name]/[Station Name] or [Bookmark Name], or "Nearest System/Station", to save you opening the Galaxy and System maps, particularly in tense situations, or to take the place of a docking computer just for the novelty of it, except that the co-pilot would also do the docking request. You might also be able to hire and train an NPC gunner like you do an SLF pilot, so that they can use gimballed weapons to varying degrees of efficacy, while you just focus on manoeuvring, but that would be seen by some as the dreaded "easy mode", or "lazy mode" *sigh*. Other NPC crew would just need to look like they're doing something useful, like the mechanic in the engine room tinkering with things, or the security staff keeping your imperial slaves in check.
But aside from some basic functions, I think that mostly the NPCs should be interesting company, whether that means being annoying or pleasant, panicking during a crisis, reassuring you or reminding you of things, telling stories, giving their opinion on the current news, asking for a pay rise, constantly complaining, clashing with other NPCs or forming friendships with them, threatening to quit, etc. I'd prefer if for the most part they'd make it about bringing some life and character into the game, for those who want it.