This is a fantastic question, and one I’ve been excited to answer for literal weeks.
There’s a concept in video game development behind creating narrative between two dividing factors: Agency, and Environment.
“Agency” here being defined as the connection a player has with his or her surroundings, NPCs, and the weight or emotional attachment the player has with specific characters over others. Environment is generally defined as the environment the player inhabits while playing the game, as well as the narrative being driven by set-pieces and tableaus.
It could be said that agency is “active” narrative while environment is “passive” narrative: both require the player to have an amount of input to garner feedback, but the storyteller has the ability to engage the player in agency “actively”, instead of environment which cannot engage the player actively (in most situations).
These concepts are indicative of great interactive storytelling, and some of the main concepts that separate games from less interactive bases, such as books or movies.
Clarifications of terms aside, let’s jump into the actual question, to which I applaud the questioner by including the term “illusion and appeal of vastness”. What we are looking for, as writers, storytellers, and GMs, is to create that illusion, and have it meaningfully be an illusion, without having the players feel pigeon-holed into linear story concepts. The greatest challenge, by far, is creating and maintaining this illusion for as long as possible.
I’ve found the greatest methods of creating a “vastness of space” so to speak, or merely its illusion, is to create player agency first and environment second. Allow me to explain.
The best methods of creating an illusion of scope is to first make the player feel very small, and rather insignificant. In the reality of all things, people are small and insignificant in comparison to the endless voids of space, so this shouldn’t be a concept too foreign for players to recognize and accept. Conversely, the player must feel empowered and significant to enjoy their fantasy setting, unless the GM is running a very, very realistic game, and herein lays the beginning balancing act.
If the players start on one planet, which I will refer to now as Terra, then give them plenty of things to do on Terra. They defeat X gang, destroy Y world-ending terror, and so on. At the end of your “Terra” session, arc, or campaign, they’ll be significantly stronger and feel like a significantly bigger deal, perhaps even actualizing this by being “Heroes of Terra”. Then, as it is a space-centric game, give them a spaceship.
Imagine the player as a fish in a fishbowl, constantly feeding on low-level thugs and monsters and sessions until it grows to be far too big for that fishbowl, and recognizes itself as a “big fish”. Then, once the player has reached “big fish” status, move it to a tank, and repeat the process.
Once they have a spaceship, have them realize that their spaceship is incredibly small compared to other spaceships. Perhaps the fastest (Millennium Falcon), has the most heart (Firefly), or has the potential to be something great as it once was (Battlestar Galactica), but inherently smaller, and inherently theirs.
Now I’ll begin the concepts of agency, and the illusions behind environment.
Their ship is small, no getting around that. Not yet at least, as they haven’t grown within the ship to have it become a stronger ship, or in our example, one that can travel further. There are hundreds of planets, perhaps thousands, in the section of space you inhabit, but it is simply impossible for you to reach them yet, because your engine doesn’t have the “Magical McGuffin Capacitor” you need.
Lucky for you, you can in fact reach these three planets, all of which may have what you’re looking for in that story seed that brought you into space in the first place.
Now you, as the GM, have given the players scope, the universe is huge, environment, three different areas they can explore to better themselves, and agency, their ship is their ship. The curtain is in place for an epic space opera of growth and experience for the players to fill into, successfully cutting your work from “Hundreds of planets “down to “These three right here”.
From here, you can work the agency angle or the environment angle. As I prefer agency, I’ll describe that method first.
In the angles of agency, you could have the ship they now inhabit come with a crew of nameless, faceless nobodies, but where’s the fun in that? In every science fiction space-traveling epic I’ve ever seen or read, the team gets to assemble their crew, handpicking only the best, brightest favorites to accompany them on their mission to the stars.
Each of these crew members will have a home planet, a backstory, a general motivation, a problem and a solution for their personalities. The PCs might investigate this and move forward with them, or they may completely ignore them and never care.
As a GM, on the preparation side of things, you could write story SEEDS for these characters, and may divulge a little of their story to each PC based on the PCs, but it is not necessary to create full biopics for each NPC, at least until the PCs show interest in them.
Additionally, each of their home planets has to be VERY FAR AWAY from where you currently are, again reinforcing the idea that there’s more out there, which the players may reach at one point, but are currently a little too small to attempt.
The players would now have many incentives to explore the vastness of space (Which you’ve convinced them is vast), being the themes of progression, betterment, your campaign focus, and the agency placed within their newfound friends, all of which you can choose to explore or not explore, but have a foot in the door as far as preparation work goes.
As far as environment goes, I believe there to be multiple methods of telling a story about space without having the players lament their gravitational pull to certain planets. For example, space can be terrifying. Make it terrifying! There are twenty planets in the far distance if you jump this travel route, but I hear each and every one of them is a manifestation of your worst nightmares, exponentially worse than the last. Might not want to go there yet; that is, until you have a reason to rescue something from there…
Space can be beautiful. Make it beautiful, and also make it ugly! The places you’re standing on now are so different, so colorful and so amazing; there are more planets like this in various directions. There are also, however, many planets filled with lawlessness, disgusting moralities, and terrible environments. The players may want to see the beautiful, they may want to see the ugly; by giving them a probing “star-map”, or other such survey of their interests, allowing you to cater to them accordingly.
Perhaps those three planets you created in the first place could be “templates” of these concepts, giving the player a taste of what they might find in each such area, as well as the dangers that lurk. The possibilities are endless.
Or are they? The possibilities do, of course, have an end, but that is something you never have to tell your players. Should you create the agency, the environment, and properly gauge your player’s interests, it should never come to running out of ideas, or having to prepare too much.
A good player has a suspension of disbelief as far as what they can accomplish based on their character and the rules you pose to them in the environment you create. The balancing act comes from keeping this suspension of disbelief right in front of your very entertaining curtain, so that the works behind it never seem as interesting.
After all, there are planets to explore. Who cares what you don’t have if you have so much in front of you?
