I have been working on this setting for some time. It came about from an enjoyment of A Princess of Mars, the kitchen sink fantasies that made-up of 80s cartoons, and years of playing (first edition) Gamma World in an equally gonzo way. One of the most influential factors in creating this setting was reading the early He-Man mini-comics, as noted here, in the second paragraph. It was just one world of many, that was ravaged by a prolonged war, and the protagonist - the evil skull-faced sorcerer Skeletor - was just a petty despot, who is not just trying to take over Eternia, but also what to find a way to get off world, so he could takeover more worlds. This not just played-out like a generic Sword & Sorcery adventure, but was also full of Buck Rogers-styled technology and adventure. I really like this premiss, and I wanted to take it to the next level, and build a whole setting out of this.
So in the universe that I am working on, the galaxy was once the home a galaxy-spanning civilization, who developed an understanding of science and physics to a degree, where they could alter the workings of reality. They made earth-like worlds - with full gravity and atmospheric pressure - on the smallest of moons, and even on layers of gas giants. They linked worlds with a network of transit gates, that allowed people to step into another world, like it was in another room (like the Stargates, but without the fancy water show). The quality of life (along with some posthumanist manipulation) was as such, that people could live for centuries with youth and vitality. They could even make major changes to their environment, without any real effort.
It was glorious, and beyond what we know as being idyllic, and it lasted a long time... until it fell apart from the inside. The reason for this was lost to legends, when the galaxy erupted in chaos and warfare. The initial battles played-out like old mythologies, where the gods waged wars in the havens, and thundered armageddon onto the worlds! Centuries has passed since the conflict ended, and the galaxy was thrown into a state of barbarism. Pieces of technology are still lying around or hoarded away, but a far too few who have any understanding with how they fully work, and for most, figuring out their functions requires a lot of experimentation. There are still people who use and maintain these artifacts as jealously-guarded family heirlooms, but production of new parts are limited, if at all. Some of the artifacts still in use are airships, gates, personal force fields (as noted here), and energy weapons. Some of the systems that allowed people to toy with reality are still in affect, but they are so complex and tricky, only few, if any, have any discipline with it. Although a product of a forgotten science, those who know these tricks are called "Sorcerers" or "Wizards," and their control over it is called "magic" or "Sorcery." As they have the power to control the fabric of reality, they are rightfully feared by those who have no understanding of it. Many of these systems were damaged, and can be harmful if not triggered right.
Most worlds are in primitive states, as the conflict have been enough to ruin their delicate ecology, or reduced them into barren wastelands, and most of the great cities have been laid to ruin. The use of reality-distorting weapons have had a mutagenic effect on life, with a lot of mutants and beastmen, and many worlds have been altered in strange and exotic ways. There are a few worlds that have been left mostly unscathed, but they survive by being sheltered to everything (and everyone) else. In the chaos, there are no shortage of despots - many of them are Sorcerers - who are trying to build their own empire.
Those who are brave enough, and strong enough, can explore the galaxy. Planet-hopping can be done by airship, and each world has its own potential for action, adventure, exploration, survival, romance, mystery, political drama, and even empire-building - what ever the players what!