my issue with promotion is that is way too *** rare. It should be a mechanic in the early game that literally begs you to fill out your officers to the max. But instead it just appears once in a blue moon, and I mostly associate it with having the option to have a new officer when I already am on max officers...
Here's how I imagine this working...
Levelling up officers should be easy, but they leave you, eventually. They retire, get bored of working for you, or start hating you, because for eg. you've lost too much crew. Promotion along more common appearing of officers would then serve in you replenishing the officers you lost. Now that I think about it, officers should also be allowed to randomly die if their ship gets destroyed.
This system sounds like a pain in the ass. I can already imagine all the people trembling in their shorts going like "but, dude, what about my 10k hours of investing into perfectly tuning my officers, and all the story points I've used with 0% bonus experience to give them elite skills". I say, we fix that too...
Make the officer system work like this... Training them and mentoring them gives you bonus experience, including for stuff like giving them elite skills. Retraining them doesn't. And officers will eventually leave you, so you'll have to keep recruiting them. On top of that put way more mercenary officers around the world. This way, instead of it being like "okay, so I have this static amount of 10 officers that are going to work for me for the rest of my immortal life", you'll go like "okay, across this playthrough, I had about 100 different officer builds. I'm getting pretty good at this".
This way players will be forced into constantly changing builds, or at least be allowed to go with various different fleet compositions. Nowadays the way this works is that you either stick to your high tech brigade or low tech brigade, or missile brigade, or cookie-cutters brigade, or otter brigade. Or you pay ridiculous amount of s-points to get new guys their elite skills and then go bankrupt, cause gaining s-points slows down to a crawl in the late game, unless you are farming Remnants (this should be somewhat fixed too, but it's probably going to really complicated, so the game will just stay sad). Or you just rely on cores. Which is also random. And you probably have officers on top as well...
I say, make the game have officers be chaotic and random, but also way more forgiving, in that it doesn't matter what skill you pick. This guy is going to abandon you in six months, or you're going to fire him anyway, cause you want to hire someone else.
But eh, this is probably too much balancing and *** to program. So, let's just forget I said anything.