Stories are about people. They may look like mice, or ants, or tree-powered aliens, but at heart they are still people. People, and what people do or think or experience or overcome, has to be at the heart of the setting. Let me start by rolling out a caveat: there is a fundamental difference (to me at least) between developing a setting for my own creative life and one for others to play in. I've spent the last two years trying to thread the needle on this, with what feels like increasing success.
The biggest reason to adventure in the settings of Star Wars or Lord of the Rings or the MCU is that the characters there are ones you know and love so much. That is also their biggest drawback as RPG settings. The best stories in those worlds have already been told. Your character is never going to be as heroic as Captain America or as smart at Tony Stark. Your hero is cool and all, but I mean Frodo carried the One Ring into the heart of Mordor, so... and Luke Skywalker blew up the first Death Star with his targetting computer shut off, and that was before he basically single-handedly overthrew the Empire. But, I mean, your character is doing cool things too, I guess.
It's the double-edged sword of the Forgotten Realms. People who love the setting seem to love it because of Elminster and Drizzt. But, your wizard is never going to be Elmnister, and your ranger is never going to be Drizzt.
For Stalwart Age, I tried to solve this by having my signature character Doc in the background. and not really all that powerful in the grand scheme. He's old, and semi-retired, and off in other realms a lot of the time. He's not Superman who is there to solve everything before you even realized there is a problem, and oh by the way I made you an omelette in the extra .24 seconds I had after saving the world and foiling the bombing of Metropolis.
For Shards, my own characters are not very cool. They're struggling level 2 or 3 guys just scraping by. They might run into your characters, but they are just as likely to run from your characters. My characters are not saving the galaxy this week; they're just trying to get by.
All of that out of the way, your setting will need to have a few 'major players' in place. These are the handful of characters, at least out of the gate, who are going to make decisions that have a trickle-down effect on the rest of the setting. Back to my foundation in acting... characters have three levels of goals: super objectives, scene objectives, and actions. A super objective is the one 'big thing' a character really wants. In Shards, Eno the Prime Director wants to slowly allow life to atrophy so that only machines remain. Nisa Montrel wants to establish the most powerful guild in the region. The messari want to find their way back to this realm. On a lower tier (where I'm playing and writing), Vex Kalar wants to restore honor to the Centurions (if it ever had that honor to begin with). Sky Stalwart wants to go home (and for him, 'home' includes traveling back in time 814 years, so that one might be a bit tricky).
Once you know what your characters want, you can work out at least one 'scene objective'. While I've always defined this as 'what your character wants to do before you'll leave the stage', it's more loosely seen as a medium-term goal. How is this thing going to help me towards my larger purpose? Hmm. Well, Eno needs to establish his power (done) and put infrastructure in place to support the changeover. That's what he's working on. It's always 'infrastructure week' on Banquo II. Once in a while enough sentients break the terms of their contracts, and he can justify vaporizing a bunch at once, which gives him mechanical warm fuzzies. Nisa knows that she needs to start turning a profit quickly so that she can build a cushion. She also needs to secure the territories she already has and ensure that her name is established. She can't helm the biggest guild until she's at least got a stable guild. Vex needs allies first. Sky needs resources first. I can't get to my big goal unless I accomplish this smaller thing first. That's where my focus is today. Then actions are the moment-by-moment choices. Right now, this moment, do I capture that guard and interrogate him, or sneak by him, or shoot him in the back? Not sure. It depends on which one is most likely to help me towards my medium-term goal. Or, even better, is this a short-term setback I'm willing to endure because it might ultimately help me to achieve my goal? It's part of what I love about Mandalorian; he is continually accepting short-term sacrifices to his larger goal because he believes that the sacrifice today will pay dividends later. I remove my helmet today because I want to help my friends, even though I know that my long-term goal of helping to re-establish Mandalor might be harmed by it, but I guess I'll have to figure that out later.
Some of the best writing advice I've read (no idea where anymore) is to create a character, throw as many problems at them as you can, and then see how they react. Another thing good actors know: acting is not about 'acting' so much as 'reacting'. You learn as much as you can about your character, and then you just listen and respond to what's happening around you. Those who are really good at improv tend to develop strong, one-note characters quickly and then react in real time as those characters, with often humorous but fully in-character responses. My 'Karen' character wants to see the manager, and nothing is going to stop her from seeing the manager (except when Heather texts me, and then everyone has to wait while I answer this text, because HEATHER). The manager responds, but he only has ten minutes left on his break, and he REALLY wants to finish his toasted ham on rye before taking forty pallets off the truck. Like, that sandwich is everything to him. Hilarity ensues.
In some ways, world building is about putting as many impediments on the road of your setting as possible, and seeing how the various characters respond. Let's take Nisa. Building a guild is hard work. The labor is difficult. There's a lot of competition. There's always sketchy stuff happening behind the scenes that she'd rather not be part of. Nobody really respects a female guild master who got her money from her daddy. She's pretty, which pretty people will likely tell you is a blessing and a curse, because it is hard to be taken seriously when you're pretty, and everyone is attracted to your prettiness and not to you necessarily.
But as the writer you also have to know your characters. You have to try to do the whole walk a mile in their shoes thing, because then you end up with interesting choices. Nisa is invited to a high-end guild party where several heads of families will appear. This could help her establish her name, or could weaken her standing if things go poorly. Can she find out their motivations for inviting her? Once she finds out, does she go? If she goes, what does she wear? Does she play down her prettiness and try to be taken seriously in some formal business attire, or does she try to weaponize her prettiness against her adversaries? I'm not sure. I'd be interested to see what she decides to do.
If it's obvious what a character would do, then it's not a meaningful obstacle. One of the rightful criticisms of the most recent Star Wars trilogy is that Rey has things too easy. She doesn't really have meaningful choices to make. Her destiny is set out before her like a connect-the-dots. To pile on Star Wars a bit, Episode I starts with a trade blockade... but why? What exactly does Naboo even trade? Architectural magazines? High-end fashion? Silver starship paint? Why is it so important? I literally have no idea.
If you set things up with genuine obstacles, your NPCs will have interesting choices to make as the game goes on, and that can almost guarantee that the PCs will, too. Some of my favorite writing moments are when characters surprise me. I wrote a draft of a novel maybe ten years ago that was never finished, where the main character was a half-goblin who was just trying to get by, but he then got a cool pet who happened to be a pocket dragon. He loved the dragon. It was the best thing that ever happened to him. But then he learned that it actually belonged to a princess and had gotten lost. The last moment of the book I wrote was that he let the dragon go back to her, because it was hers first. It was the moment he changed from looking out for himself to looking out for others. I had no idea what would happen on that last page until I got there, and I straight up cried for ten minutes when he did it, because I was so surprised and proud that he made that decision. I had no idea he'd do that until he did it on the page as I was typing. That was the moment I knew that I had written a pretty clunky first novel that would probably never be published, but it was also the moment I knew what it was to be a writer. It was the first time I let the characters tell the story.
I encourage you to do the same. I'll bet your characters will surprise you.