|The Star Wars presents its frame of reference|
A good setting, whether for a sci-fi story (or movie) or a role-playing game, needs a frame of reference. This is a central element of the setting that affects many, though not all, of its other elements. This also allows you to fall back to the frame of reference when you run out of ideas or have to improvise.
A frame of reference is neither a theme per se nor a mere plot. It is the combination of plot, backstory, and themes - the glasses through which you see the setting or franchise.
For example, both Star Wars and Babylon 5 have excellent frames of reference. Star Wars (the original trilogy) starts off with scrolling text describing the Empire and the Rebellion against it. Moments later, we encounter an Imperial Star Destroyer chasing a Rebel Blockade Runner. Almost any world or other setting elements in the original Star Wars universe has something to do with this civil war; most have Imperial presence. Others have hidden Rebel bases. When designing an RPG adventure in the original Star Wars universe, for example, you have a frame of reference to work from - there is the Empire and there is its war against the Rebellion. It colors almost everything. It also serves as a terrific starting point for plot and setting elements.
The same goes with Babylon 5 - its pilot starts with an intro speaking about the Earth-Minbari War, then proceeds to have a plot directly connected to it. While many Babylon 5 plots have little to do with the Earth-Minbari War, it is always in the background and serves as a launching pad for stories in many occasions. Son enough the story moves on to the main frame of reference, which is the Shadow War.
So, when designing a new setting, think of a major frame of reference that defines your setting. Was there a massive War recently? Is this a post-apocalyptic setting? Or did Earth discover faster-than-light travel just recently and is now exploring the wide-open frontier of space? Either way, a frame of reference helps you build a good setting.