Starships.

So this will be a game with a multitude of inhabited planets orbiting quite some stars, and somewhat detailed crews to push around… right, there ought to be spacecraft.

 Of course in such a title said spacecraft should be as modular as possible, not only because this allows for a very detailed damage system and more interesting flight-physics, but also because the players can build their own starships from modules. (also the NPCs build their ships from modules, what should be good for quite some variance), but wait, there is more.

I personally deem not space sim complete, when there is not a cockpit view. If you can expect it from semi-historical flight sims, you should be able to expect it from your regular space dogfighting game, even if it makes not that much sense.
From this point on, it's pretty trivial (from a technical standpoint) to allow free movement inside the ship, pretty much like we know it from Silent Hunter 5. But this time in space, and with Role-playing mechanics in it. Interacting with your crew should be one of the most important parts of the game.