In 1977 the first role-playing game for playing games in space came out. Like the other major table-top RPG system developed in the 70s, Traveller is still with us albeit many revisions and game companies later. A major part of how Traveller has endured so long, is its incorporation of a series of basic ideas about what makes a space opera. These components of the space opera incorporated into rules as much as setting mean that any of us can recognize and recreate the specific story we want to tell within the game. However, this tells us nothing about creating space operas in Traveller or any other system, since as I say this is all up to you.
There are certain core concepts that make a Science Fiction story a space opera. First and foremost among these is normalized and regular travel through space. This can be as realistic and understandable as the basic intra-solar travel in Leviathan Wakes to the vague and overpowered “Hyperdrive” of Star Wars. Whether or not the story takes place on a space ship is not as important to the idea of the space opera. Dune is most certainly a type of space opera, however little to no time is actually spent on the bizarre ships of the Spacing Guild. Narratively, serving as the crew of a space ship or a group who must stick together for vast distances of space provides cohesion. This is even more true in table top games than literature where conflict between characters is observed, not enacted.
Once you can travel regularly through space, it becomes assumed that there are many habitable and inhabited worlds.For the most part, space opera requires frequent Star Trek-like planetary investigation. Discovering new species, may or may not be important, but discovering new worlds with distinct cultures or characters is. The new Battlestar Galactica somewhat dispenses with this particular idea. The inhabited worlds of that story being the previously departed colonies of the crew and the occasional habitable worlds they discover, which are already occupied by Cylons. From a narrative perspective, he main purpose of the planetary investigation is to provide conflict off of the ship itself. The paranoia of Battlestar Galactica is a desired story trait, which is how it benefits from the cramped ship quarters and intra-party conflict. Unless you want your game to develop this sort of feeling, providing planets with their own external source of conflict is important.
Beyond these two basic concepts, which should be followed for any space opera creation, the details are often individualized and left to the game master and his or her players. After the Original Series, Star Trek relied heavily on occasional Holo-deck episodes to flesh out character interactions and revive some of the goofy aspects of TOS. Asimov and Herbert both essentially built galaxy spanning empires entirely composed of humans. A human only universe would never quite have worked for the story told in Mass Effect or Star Trek (I tend to think that Star Wars is the same story even if you take out all the non-humans). Depending on where you fall on the idea of Doctor Who as space opera or not, Time Travel could even be a central idea.
Beyond the idea of spaceships and other worlds, other aspects of a space opera can take on a central aspect or serve as window dressing to suit the tastes of a group. In a space opera story where the Terminator films were canonical, the end results of the human-robot war would be central to the ultimate history of mankind. Did the robots win and humanity is on the run from evil robot enemies, à la BSG? Or did humanity win and artificial intelligences are now illegal such as in the Foundation novels? In a world where robots are as ubiquitous and ignored as they are in Stars Wars, we can safely assume that sentient artificial beings never engaged in large scale revolt.
As a Game Master, it is important to consider all of these factors as you design your specific world. Think about the role you want your player characters to inhabit in the era of the story, as well as the sorts of science fiction your players are most prepared to swallow. If your players like realistic human centered stories, consider leaving out intelligent aliens entirely. On the other hand, if archaic Vulcan-Romulan political relations are everyone’s favorite part of Star Trek, you could create an entire space opera in a time before humans were on the scene, something a novel or TV show would have a hard time getting away with. Traveller has been successful because, like the space opera genre, it is open ended allowing for the specific interests of individual groups to take precedence. As long as you maintain the dual core of spaceships and planets other aspects of the space opera should be yours to choose. Though maybe leave out the time travel thing.