A question I think about sometimes for writing projects is 'what is the minimum viable chekov's gun?'. Like, if you conspicuously put the proverbial gun on the mantlepiece in act 1, what's the least use you can put it to that will pay it off? Can you put the proverbial gun on the mantlepiece and pay that off without firing it? Without anyone getting shot? Not necessarily in a subversive way ('haha! the gun is loaded with blanks!' or 'haha! the gun isn't loaded, but a secret message from the wife's lover is concealed in one of the chambers!') but in a way where someone noticing, picking up, interacting with the gun pays off something significantly less dramatic than violence or the threat of violence?
Today I'm thinking about this in the context of alien megastructures, which apparently I'm just coming up with as settings for everything these days. If I have a story set in a cool alien megastructure because that's a cool place to put a story, how little can I get away with explaining or revealing about the alien megastructure and have it still be enough that people feel the alien megastructure is more than just set dressing
Idle thoughts, conversation starter, tell me about cool books/plays/stories that have deflationary payoffs (which, I guess, implicitly there's a spoiler risk in the comments, I don't mind being spoiled on stuff but I know some people do)
I second the comment suggesting thematic relevance; whenever I need or want to include a certain scene, conversation, character, etc, but need to "pad it out" somehow so that it doesn't feel sparse or out of place, I ask myself "how can I make this thing reinforce the theme?" Or, "how can I use this to support the characterization of my protagonist, the people around them, or the world they live in?"
According to the theory of Chekhov's Gun, every story element should have an answer to the question, "why was this included?". Sometimes the reason is to set up a more specific question, and sometimes it's to answer a question – like Chekhov's loaded gun, which asks "when will this be shot?" and may later answer "how will this violent conflict end?" Personally, I think "because I thought it would be fun" is often a good enough answer, because entertainment is the whole point of much media in the first place. But the more reasons you have to include an element, the more efficient, effective, and interesting the element is.
As @garak said, the alien megastructure could contribute to a theme of "living in the shadow of something large and unknowable, or completely alien". Another theme could be "being a small part of something much larger than you", either for good, like contributing to an underground resistance, unable to see the full effects of your actions but praying that your piece is helping to make a difference; or for ill, like being a cog in a capitalist machine that will grind you up and then keep going. There's also "being trapped in a maze of options" or "an emptiness so large that nothing could ever fill it up" or "who cares about the big things, honestly, it's the little close things that matter" or "stop blocking out the world and leave your comfort zone for once already". Or even a couple of these themes on top of each other!
The megastructure could contribute to characterization. Maybe the protag's mom is so controlling because she grew up somewhere much smaller, and in this big mysterious place she can't help but try to carve out a home that is hers to know and control in its entirety. Maybe a best friend's plot-contributing technical skills were learned during their work maintaining the megastructure's anti-gravity lines. Maybe the government is prone to excess, huge flashy projects that don't actually might life better – or maybe it used to be capable of such great vital good but is currently too scattered and polarized to muster the effort.
Some questions the megastructure could ask or answer: "Who built it? Why was it built, and is it still fulfilling that role? Why do people live in it now? How does it affect day-to-day life? What is it made out of, and where did those resources come from? Do people still have access to the level of technology/resources/labor necessary for such a project? If not, why not? What do the people who live here know about and think about aliens? How do people travel? Where do they work and where do they live, what are their jobs and their hobbies? Why are these people here, either in general, or the specific characters we're focusing on?" And any of those answers could then, in turn, be relevant to theme or characterization or plot or more questions and answers.
It's hard to say where the line is, of how much of this you need... When I'm reading a story, I'm happy to see any kinda cool shit, even if it's fairly irrelevant – though I'd be disappointed if a cool thing was mentioned, but then I didn't get to actually see any of its coolness. But I do also find it extremely satisfying when story elements become relevant over and over again, in different ways, from different angles, asking or answering new questions each time, punching you in the gut with renewed significance and context.
So I suppose my answer would be: If you want to put something really cool in the story then fucking put it in there, no matter what - but keep an eye out for opportunities to tie it in to the rest of the story and do so wherever you can, whether that ends up being only once or twice, or in new ways every single scene. It's just even more cool that way!
