This yearâs been stressful, so Iâve fallen a little behind on both. In the manga, Iâm in the Wano arc, and theyâre on the roof fighting Kaido and Big Mom. Iâm a little further behind in the anime, closer to the start of the Wano arc.
Thatâs a complicated question. Especially since One Piece is such a massive story that you could found a whole series off inspiration from almost any of the islands theyâve visited. I also have a method of building stories that Iâm not sure is common, at least Iâm not sure Iâve heard an account of something similar from another writer before, so it might be hard to give advice that works for you?
I describe my method as some combination of âfanfiction with a whole hecking lot of extra stepsâ, and âa Frankensteinâs monster amalgamation of a storyâ. I pick one or more stories that inspire me at the time, gut them for parts, strip a bunch of characters (that may or may not all be from the main inspiration) down to one or a couple things I like about them, then rebuild their personalities and the plot by packing on details from other random inspirations and finally smoothing everything over with a thick varnish of logic to make sure everything makes sense and fits together cohesively.
I do have a story idea with One Piece as a founding inspiration, but Iâve been working on other things so itâs kind of still in the early stages and I donât have much to talk about, so Iâm gonna use my current work in progress as an example instead.
My current work in progress was founded on My Hero Academia. Specifically, âDeku inherits All For Oneâ. So I gutted the story down to a framework of âDeku inherits All For One, steals a whole hecking lot of superpowers, becomes the number one villain, and takes over the worldâ. Gutted Deku down to âpowerless but determinedâ, then slapped on a bunch of new inspirations and logic to rebuild him into the protagonist I felt my new story needed.
I wanted Navon Yandell (new Deku) to be older, because I wanted to write for an older audience, and I hate school with a passion. So I wanted him to be graduated already, but also undereducated, so he could learn the magic system from scratch alongside the reader, which meant he had to have basically cheated to graduate, so I made his parents top heroes nobody would oppose (throwing in Yagi and Inko as gutted placeholders, as âgood-hearted but dense dadâ and âsmall mom whoâs proud of her boyâ) and filled the school staff with corruption. I needed other students to contrast Navonâs patheticness, and eventually be his allies, so I threw in Iida (gutted down to âsmart, awkward boyâ), Ochako (gutted down to âfriendly, heroic girlâ), and Katsuki (gutted down to âangry, strong, gifted boyâ). I knew Navon going that long without powers and getting a diploma he didnât deserve would probably make him cynical and depressed, so I threw in some of that, and some self-deprecation, but balanced it a bit with self-awareness, goofiness, and subtle wholesomeness. Threw in some other things I liked and that I thought would make him more interesting and competent. Luffyâs (from One Piece) social genius and difficulty communicating it, and his sheer willpower to beat the hell out of anyone who hurts his friends, Moxxieâs (from Helluva Boss) low self-esteem and self-doubt, and hesitance to cause harm (like in the case of not wanting to kill a human family), yet willingness to speak up and complain to his boss about his antics, but also a bit of Blitzoâs (the Helluva Boss in question) silliness and lack of regard for personal space, but not too much of that, some of Johnâs (from Homestuck) passion for things heâs not particularly good at and tendency to get distracted by the goofiest, most pointless stuff, Batmanâs (from Batman) rigid determination not to kill criminals, the small anger of a chihuahua, Edâs (from Fullmetal Alchemist) height complex, and eventually developing something like Edâs studious, scientific mind, Bellaâs (from Twilight) determination to hold onto a love interest who has hurt her because she believes in the good in him, and determination to get her happily ever after, and love of vampires, my own (from me) fear of being hated and thought of as a demon by people who donât know the real me (being LGBTQ in a fundamentalist Christian household), some of my own questioning and intrigue concerning religion, some of Dioâs (from Jojoâs Bizarre Adventure) villainous presence and confidence, and Iâm sure there are more I canât remember off the top of my head.
I could, and shouldnât, ramble an entire second book about how I came up with everything in the book Iâm writing. Sorry I rambled as much as I did, but I hope something I said might help. 
For how to lay all this information out for yourself, yeah, just write out the worldâs history as if youâre transcribing a textbook for it. In order of time, importance, or order in which you think of things, it doesnât really matter. It doesnât have to be publishable, and might be hard to share anyway because itâll probably be less of a story and more of just an account of a world, but having a publishable story isnât the point of laying it out for yourself, the point is indulging your own interest in it and getting all of your thoughts straight.
Also reminds me of one of my stories. Itâs fun exploring characters searching for true self and family.