I've been reading through my old Talislanta 4th edition book again. Talislanta, for those who don't know it, is a long-running and rather fascinating fantasy setting which is especially noteworthy for its multitude of fantastic races - none of which conform to the "standard" fantasy races such as elves, dwarves, or even humans. Instead, each region of the world has its own unique race with its own unique culture. There is no dominant "baseline" race which you can compare the other races to - just a huge variety of idiosyncratic societies with their own cultural quirks. (You can freely download much of the game line here.)
And I was thinking that you could do something similar to Talislanta as a science fiction setting - not replicating the Talislantan races and cultures, but using the same basic principles in setting design. Here is how I would do it:
Let's say that Earth colonizes a large sector of space and
settles it throughly. Human genetic engineering becomes very common,
both to adapt humans to the various environments they encounter (often melding the DNA of native species into their own), and for
general human enhancement purposes - or just for reasons of fashion,
alternative lifestyles, or even attempting to mold humanity for
particular political or philosophical ideologies. Humanity effectively
splinters into innumerable subspecies. Psionics are also discovered and
become widespread - also thanks to human genetic engineering.
happens to destroy the stargate system, which also
destroys much of the technological and industrial infrastructure of the colonies.
They are forced to survive in isolation for more than a thousand years,
though some sublight travel between the stars is eventually done by
especially daring cultures. As a result, the different subspecies of
humanity diverge further, and by the "default" time of the setting there
is no longer a recognizable "baseline" humanity - each colony
world has its own distinct subspecies of humanity (and possibly
Techology largely declines among the settled worlds, although a few of them maintain to cling to higher tech levels. Then someone figures out how to use psionics (which didn't decline as much, and largely takes the place of "magic" in other settings) to physically move to the Plane of Dreams - a parallel world of our own, but where space and time are more fluid - and actually travel to a different colony world, as no life support is needed in the Plane of Dreams and dream travel can exceed the speed of light. Eventually, powerful psis learn how to transport entire caravans or ships to the Plane of Dreams and back again, allowing interworld trade even with fairly primitive technology. It turns out that the transition is easier when one is far away from other people, as the dreams of sapient beings cause disturbances while moving into and out of reality. Thus, caravans have to travel to fairly remote regions to make the transitions, while ships can simply sail out to sea. Spaceships have it even easier (though their size and cost still makes it difficult) - although not if they are too close to the stars, which tend to be surrounded by especially tempestuous and alien dreams (which would imply that they are inhabited, somehow...).
Anyway, there you have it - numerous different worlds, each with its own human variant, tech level, and its own unique culture. Travel is common enough to make it feasible for typical adventuring types, but dangerous and strange enough to be exiting, and slow enough to prevent interstellar monocultures. Each world is thus a new world of adventure.