So, I'm thinking about space ships. SHOCK.
I'm thinking about RPGs, too. One of the cool factors with modern RPGs is that you can design your character both aesthetically and statistically. As the game goes on, your original choices weight how your character evolves - what skills you pick, what armor you equip, how you approach various challenges. Each of those is, in itself, another piece of player-generated content.
True, choosing a sword or wand isn't quite as "pure" as shaping a character's face and carefully weighting their statistics. But it is still a player choice that customizes the world to some extent, so it's still player-generated content.
So, I was thinking about starships.
Let's create a game where you design starships. You put together the modules, the layout. You choose half a dozen crew, put them in various rooms and give them various ranks and roles. The starship itself has a specific mission - go to Jupiter and mine Jupitite or whatever.
That's creating a character.
In the course of an RPG, the primary shaping force your character faces is the onslaught of endless waves of enemies. We don't have enemies in our starship game. Instead, you face an endless onslaught of interpersonal drama. You literally bring the monsters with you, because your crew is the source of your difficulties. The longer and more stressful the voyage, the more severe and dangerous the drama is. If you fail to deal with it, it'll damage the relationships of the characters (making the next drama worse), or even escalate to violence against the ship or the crew.
So you deal with it by directing the crew to use specific parts of the ship. If Anna and Bob are fighting, you can send them both to their quarters (or just one, if you prefer) to let them cool off. That's equivalent to "attacking". You can repair relationships and sooth interpersonal bad feelings by having the crew participate together in a recreational activity. That's equivalent to "healing".
Sounds simple, right? Well, not exactly.
Just like an RPG, as you spend time in space your ship develops. The characters begin to customize the modules and get used to them, allowing you to "level up" a module's performance on a specific character. Retrofits and software upgrades allow you to alter and upgrade the kinds of actions a module allows. You can even add in new gear, in the form of replacing modules, redecorating, planting new plants, etc. All of these serve the same role as skills and equipment in an RPG. And, of course, you "level up" (gain customization points) by successfully solving interpersonal drama events - that makes sense, since you come out of it understanding your crew better, and can therefore make your ship more comfortable to them.
It's a relatively simple idea. At the beginning of the flight, your crew might get into arguments over whether to have eggs or waffles. But after a year in deep space, they will all be ready to snap, snarling insults and over-reacting... even as the most critical part of your mission now needs them all to work together! How do you keep them under control?
Just like an RPG, you choose a "class" in a variety of subtle ways. For example, you might go with luxurious crew cabins so your "basic attack" is very effective. Or you could save a ton of mass and maintenance by going with a "crew bunk" system that severely limits your ability to send the crew to their quarters to cool off, but frees up a massive amount of mass and maintenance for other things. These strongly shape how you will deal with the different kinds of drama your crew goes through.
After the mission, when the ship returns home, you can reallocate the crew to other missions (after a vacation) and either dismantle or reuse the starship itself. Either way, the same crew won't be willing to serve on the same ship, so to some extent most of the leveling will be lost.