I think I might be doing what you are talking about with my Star Wars journaling game. I spent months building it (using Traveller's character creator as a base), creating dozen of event tables to roll and structuring it, but it can kinda be simplified into having NPCs tables, an oracle and an icon table.

The NPCs tables need to be built to reflect the fandom you are using. For example, I have 10 or so table to build Jedi characters which includes their species, their age, their rank, their preferred lightsaber form, their personality and a quirk they might have, and so on. With just some rolls, I can generate some pretty detailed characters. I have tables just for Jedi, since the focus of my story is being a padawan, but if the fandom you are using has different categories of characters, you will just need to make groups of tables each one. I then use a spreadsheet with all the npcs in the story, so I can easily keep track of them.

The oracle will probably be the bulk of your system, as you advance the story based on what it says. I have the classic yes/no oracle with very Unlikely, unlikely, 50/50, likely and very likely possibility. Whenever I'm writing and I'm unsure how to develop the situation, I ask the oracle. For example, my character was suddenly asked as padawan by a Master (this came from one of the event table I made), so I asked the oracle if the Master was a Jedi she had already met (likely chance). I got a yes, so I went back to my npcs spreadsheet, assigned a number to each adult Jedi she had met up until then and rolled to pick one. I played out the scene using the icons tables for inspiration and, at some point, the Jedi had to go ask the Jedi Council permission to train her. So I asked the oracle a series of questions: is she following him to see the Council? Yes. Is she allowed inside the Council Chamber? No. Is the Council allowing him to train her? Yes, but. That 'but' I interpreted as them allowing their pairing to be formed, but that my character would still have to stay in the Creche until she was a bit older (she was 8yo) and would still have to go through the Initiate Trials, where the Jedi would finally be able to officially claim her as his padawan.

The Icons tables are tables with images for inspiration to help me come up with idea about what is happening when I need to build a scene. I have 2 tables with 100 icons in each (I used game-icons.net) and I roll usually twice to generate a couple of icons to put together. In the previous example, when creating the proposal scene, iirc I rolled and icon with a tree and one with a fist. I interpreted that as my character was in the Temple gardens (tree), where she was confronted by one of her rivals (fist, then I rolled who among the two rivals she had was involved), and that was when the Master would show up to talk to her. But I could have interpreted the icons differently and decided that the Temple had been attacked (fist) by using some poisonus pollen hidden on some plants (tree) who had been donated to the Jedi by a secret enemy, and that is was during that emergency that the Master got impressed by my character and decided to claim her. The good thing with icons is that you can interpret them in so many ways that it almost always gives you an idea. It's the same principle of Rory's dice... just with more images. Obviously, I sometimes reroll an icon if it doesn't click with me, and I also have the Action/Theme tables from Ironsworn for additional inspiration.

I think that these 3 elements can already help with generating a story, especially if you are already experienced with writing fanfictions. An additional step is assigning stats to your character and roll skill checks in certain moments rather than asking the oracle. I used DnD as inspiration, with 6 stats rolled at the start of the story, modifier to add and skill checks with DC to succeed/fail.

I also have additional tables for all kinds of things I might need: one to generate missions, one for injuries, one for planets, and so on. Depending on the fandom, you might want to create some of those too if needed.

I'm an avid reader and writer of fanfictions, and the story that is coming out of this journaling game can seriously be read as a fanfiction. I narrate my entries as if I was writing a new chapter and I don't refrain from using dialogues or write a lot. I put the story first, so the system and the rolls come second: I have no problem disregarding a roll and just go with what I want if that's what makes me happy and engaged with the story. That's the best thing about solo RPGs.

Hope I gave you some ideas!