Wow, so, basically it is your world, your story, and your characters. I honestly think you should simply give up the pretense and write fantasy novels.
And how ironic that you post this here in a thread on "Player-driven campaigns and developing strong stories."
I have no idea how you concluded any of that from what I wrote. It's just bile and has nothing to do with actually rationally looking at the topic and answering the OP's question.
Here is my take, once again restated: player driven campaigns are driven by skilled players. Whether those develop into strong stories depends largely on skill of the players to personify characters and give them strong dialogue and motivations, and to make strong character driven choices within the setting.
I think it's rather odd to talk about what you can do as a GM to create player driven campaigns and strong player driven stories in those campaigns. Really, what can you do beside let players create their own choices and just respond to that? A GM can't make a player create a good story. A GM can't make a player RP well. A GM can't make a player create interesting choices that will lead to interesting circumstances. We're supposedly talking about respecting a player's agency but we're doing it from the perspective of the GM making all the choice.
I think a better approach would be to focus on what players can do to drive a campaign and create strong stories. The topic really IMO should be, "What can I do as a player to tell better stronger stories?" And I think if the thread took that approach honestly, it would reveal how weird the original post actually is. Like, can you imagine writing an adventure to be purchased by and consumed by potential players? And like how would that actually empower players? What would a player's story look like and how could you communicate it, and if you did wouldn't it cease to be the player's story? And would the player really want to know their own story? How can the player be the secret keeper?
Think about it this way. My PC's are bounty hunters working for the bounty hunter guild. Like, I could run the game by presenting the players with 10 or 20 choices of what bounties they wanted to go after. But even if I did that, I'd still have to invent the scenario. Currently they have been hired by an Imperial Prefect to investigate the disappearance of colonists on a frontier world. Now do the players want to know at the start why the colonists are disappearing? And remember, "We are bounty hunters" was a player driven tagline. The current scenario is running at like 22,000 words of notes plus maps, and most of that is documenting what happened before the PC's arrived and so therefore, what history exists for them to uncover so that they can make real choices based on their own investigation. I don't have a clue what they are going to do, but I do have to document very carefully what is there because the resulting story has to be internally consistent. You got 10 different disappearances and crime scenes and impacted families and communities that need time investment to describe. And, minor spoiler, there is actually an important element of the timing of the crimes that required a lot of work from me to get the outline of the past right. All that world building is necessary if I'm actually going to create a sandbox that also generates the big linked dramatic story that the original poster wants. And this is just for a tiny world - on little corner of a planet in a little corner of a galaxy. The idea that I could ever detail the whole galaxy at such a level that like these complex linked narratives would just naturally emerge no matter what the players did is just impractical. But even if I did, very likely underneath the hood the resulting story would be almost entirely Illusionism, which is the opposite of what the OP says he wants. The idea that I could actually have a database of millions of bounties for the players to look through is just not practical. The idea that you just do this on the fly and not end up with either high illusionism or a series of small disconnected events is also impractical.
The underlying problem is that it's not that fun if the same person who introduces a problem is responsible for solving it. I mean that's the problem with a railroad - the GM both introduces the problem and sets the solution. But the reverse doesn't work either. You can't have the player both introduce the problem and its solution because then there is no drama. Like imagine the players deciding on why the colonists are disappearing and how they figured that out and how they resolved the story if they also wrote the backstory. Imagine the players creating the setting for them to explore. One of us has to be responsible for the backstory and the other responsible for the forestory, and the only way it works out is if the secret keeper writes the backstory and the player's choices creates the forestory. Someone has to decide where the orc and the pie are, and it works best if it's the player who wants to unravel the secret of the pie. It's up to the player to decide what they do about the orc and the pie, but they can't do both because what's the fun of declaring you win? And saying that the player should be able to decide that a dragon comes and eats both the orc and the pie doesn't really solve the problem here, especially in a game with more than one player.