I've been sitting down with some old style analog pencil and paper to start outlining an actual campaign in a custom created setting. Getting the broad strokes done is easy. Pick a few sources as references for your own stuff, decide on a rules system, and put together a list of alien species and the default technologies in arms and space ships.
But then... what actually?
Creating a couple dozen planets with their own geography, culture, and history is a pointless undertaking. It would take forever and barely anything of it would ever see any use in actual play. I am a firm believer in that good campaign setting worldbuilding is tailored to the needs of the kind of activities that PCs are expected to engage with. But what exactly are those needs? This is something that has been brought up earlier in this thread. Fantasy games are perceived as more accessible for players (and GMs) because the archetype of the fantasy adventurer is well established and understood. They stab monsters, bandits, and sorcerers with their sword when they see them and get applauded for it by the masses. In futuristic space settings we don't have such an established role for PCs, though I do not believe that this means they are inherently less suited for play.
There is of course the space soldiers fighting in space war model, and I think that is pretty self explaining and straightforward. Gameplay consists of attacking or defending ships and bases, with periods of interacting with guides and informants or getting new orders from the higher ups. You go where the enemy is (often being directly send there on a specific mission) and fight the enemy until victory is declared.
But I started this thread specifically with "space adventure" in mind, with which I mean parties of more or less honorable rogues traveling the galaxy in their ship and having run-ins with gangsters and law enforcement and other exciting shenanigans. I think this character archetype is well enough understood as well, and there are loads of existing examples in fiction. But translating that into a game structure and tailoring a campaign setting to support those structures isn't quite as obvious.
I've recently been thinking about sandbox and survival games in a different context, and it had me thinking about push and pull factors in regards to getting PCs to become active on the players' own initiative instead of being handed a task.
Typically in heroic fiction, we are dealing mostly with pull factors. An inciting incident happens, it signals the arrival of a great evil or major threat, and the heroes, being heroes, are pulled towards it by their desire to see it stopped. (The Expanse being a good example of such a setup for a ragtag bunch of misfits with a cool ship who are compelled to act out of a sense of compasion and justice.)
But when you are dealing with a gang of scoundrels and other rogues who often get into trouble with the law, this often does work out very smoothly. If you invite players to make such characters, they are absolutely justified in not wanting to get involved in external crises they see on the ground of "what's in it for me?" But it looks differently when you approach character motivations in the form of push factors. Things that make the status quo of the PC impossible to maintain and forces them to do something, anything, to adress their own situation. Even when you are dealing with PCs who are lazy cowards who would love to do nothing but spend their nights in bars, this can be a great character motivation if you have something at work that keeps disrupting their tranquil stupor. In many survival games, this push factor is food. Your character needs food, and there is no food in your little cave where you can safely hide from predators and bandits. It's your character's hunger that is pushing you out of your safe comfort spot and forcing you to get food. Somewhere, somehow.
In a game with some rogues on a cool ship, maintaining that ship can take the place of hunger as the push factor that is driving the entire campaign. If you only have to pay for repairs if your ship gets damaged, and only have to pay for fuel when you fly around, the sensible thing is to park the ship somewhere safe. But when the party is losing money in maintainance costs or the ship keeps degrading by itself even when not moving, then that no longer is an option. The players are forced to go find money. Somewhere, somehow. And it never stops, it's an infinite loop that keeps pushing the characters to go on new adventures for all of the campaign.
I think this is a good starting point to begin my own tinkering with this approach, but it surely isn't the be all, end all in regards to this issue. Any other ideas how to begin formulating a kind of campaign structure or gameplay loop for space adventurers who might not have an endless pool of compassion in their hearts?