I recently read Gateway by Frederik Pohl and it struck me as having potential as a roleplaying campaign setting, so here are a few thoughts on the subject.
The premise of the book is that humanity has discovered an outpost of a deceased alien race. This outpost is full of alien ships, which can be programmed to visit a location and then return. Of course, no one understands the alien navigation settings, so people set off in them at random; sometimes they come back empty handed, sometimes they come back with alien tech and get a big pay day, sometimes they don’t come back at all.
From an RPG point of view, this could be played out in a couple of styles…
Firstly, it would be a good background for a series of relatively independent one-shots – the PCs pile into a ship, blast off at random, have an adventure and come home. This could either be done with a regular group, keeping the same team each time, or within a much larger group, where players rotate in, as and when they’re available.
For a really casual game, you could keep the players in the dark about where they’re heading each session. However, if you want to add a little player agency, you could tweak it in a couple of ways. There could be an emphasis on repeat visits using the same settings (“let’s head back to planet X; we never checked out that jungle temple.”) – this would allow for slightly more substantial plot arcs, even though each adventure would still be fairly independent.
Another approach would be to metagame it slightly. Once you know who’s playing a particular session, they can pitch the sort of themes that they’re interested in for that adventure (exploration, horror, mystery, action); they still wouldn’t know where exactly they were going, but at least they could be assured of the sort of story that they’re interested in.
With all these approaches, my suggestion would be to make the “hub” (whether a hollowed out comet, like the book, or a space station, or whatever) an adventure-free zone. Adventures take place when you set off to a destination; the hub is merely for refreshing supplies.
The alternative to a series of one-shots would be a more substantial campaign. At this point, you would want to introduce life to the hub, as that’s where the continuity will be. The players will still go on exploration missions, maybe repeating the same locations or maybe somewhere new every time, but the social side of things will occur between missions.
There could be political parties within the hub, trying to control what happens to artifacts that are brought back; there could be a criminal underworld, trading in specialised equipment to help explorers survive; maybe there’s a fee to try your luck with a spaceship and the players don’t start with enough, so they have to do a couple of favours before they can even set out.
As a final note, I would almost certainly run something like this using Numenera. Some of the recent, space-based material would feed into it nicely, but even with just the core system, cyphers and artifacts are a great model for the sort of alien tech that explorers would find.