Once upon a time, a long long time ago, the Sun and the Gravity God we're fierce enemies to each other. One day they fought each other on a duel, and though they were equals both in size and strenght, the sun won; and then he broke the body of the Gravity God into little pieces and scattered them around the void.
As the years passed, the floating residues we're drawn to those pieces, wrapping them and thus creating the planets. Nobody knows how many planets there are, due to them being orbiting constantly around the sun in different distances and planar inclinations; and creating an useful map comprehending them is a futile task. The smallest habitable planets can barely fit a house; the biggest one, which also holds the biggest city known to man, is about the size of the real Earth's moon.
All those planets share the same atmosphere. There is air between them which can be breathed. There runs the wind and the clouds, alongside birds, people and even birds: if you're outside the gravitational pull of a planet, you can use friction to "swim" in zero gravity (artificial wings, crank-powered propellers or bird carriages are also popular) A traveler may go from planet to planet like that, if he manages to lift off from the departure planet. Jumping high enough may do the trick on very small planets; in greater ones you might need a rocket, a pole or jumping from a high tower specially built for that purpose. (Note that, even though the average surface gravity is roughly earth-like in every planet, the gravitational field radius is ridiculously small compared with ours, and always proportional to the planet's size). It is wise to carry a parachute or other means of cushioning when landing on a planet.
Due to this conditions, big interplanetary vehicles are extremely rare. There are, however, some land and sea vehicles on big planets where they can be useful. The other thing that exists only in bigger planets is true night; and even that isn't ever total darkness, as the light is reflected from the atmosphere behind the planet. However, the most further from the Sun is a planet, the darker and colder it is.
People around this universe practises martial arts, inspired, at least philosophically, in things they have closer: gravity, the Sun, birds, whales, etc. The most used weapons are slings, poles, nets or javelins, which are useful to hunt both on land and on zero gravity (you can send a slingstone really, really far in there, and shooting wrapped messages around a stone or a present is a feasible way of interplanetary communication)
In other ambits, technology levels are very different from a planet to other, as there is little trade or communication between the different lands. The same goes for religion, philosophy or other fields of knowledge. There are also certain old and strange wise men and women who study the movement of the spheres and can foresee which planets are gonna meet soon, or which directions to take to get to a specific place so one may travel there.
People on this lands strangely wage war due to the impossibility of worlds to be near the sufficient time; but figthing duels on single combat is a traditional way to settle disputes, just like the Sun and the Gravity God did. From time to time, martial arts fighters go traveling the world searching for ways to improve their strenght, searching for masters, foes and adventure, and maybe searching for the magic item _________ which can _____________ and is totally essential to fulfill the dream of ___________________ of one of the characters.
So this is my current thing I'm playing with in my head, which may turn into a game setting of a campaign someday. Is it any cool? I've been wanting to do some Kung-Fu / Ranma / early Dragon Ball / Steven Universe related thing for a long time, and this might be it.