How do you properly plan for a very long story? I was thinking about One Piece and the fact it's still ongoing, and I am wondering how the author was able to plan for such a long story. Writing a single beat sheet wouldn't be enough to plan for it, and yet the story has been going on for years without alienating its large audience. How do you plan for such a long story? Is it even possible to plan such a story? What would you suggest people if they wanted to undertake such a large project?
I think the secret lies in preparing characters, the world they live in, and the conflicts or tragedies they face, with most contrasts amongst them.
Think of the „files“ detectives always have about anybody in US films, or the dossiers psychologists do have. These usually span their lifespan, at least. If well designed, these work as a kind of convincing generator.
Assuming you know „Big Bang Theory“ nerd Sheldon is known with all his flaws and twists. So following a scene today is both known and still surprising: which elements will they combine or show for the first time today from said files? How will his friends (have to) react (from their files)?
This seems to be designed or sketched so well, it even is in harmony with displaying his childhood as Young Sheldon“.
In practice, you don't.
I am pretty sure that when Eiichirō Oda wrote the first One Piece manga in 1997, he would never have thought that he would still be writing the story of the Straw Hat Pirates over 25 years later.
When you develop a new media franchise, you usually don't know if it will be long-running or not. It will depend on the audience reception whether it makes economic sense to do one book/comic/movie/season/game/whatever with a newly developed IP, four, or forty. And you won't know that until you published the first installment of the franchise. And even if you do get lucky and your work manages to find a dedicated fanbase who demands more of it, you won't know how long their enthusiasm will last. So it doesn't make too much sense to plan the story of your franchise for decades in advance.
But on the other hand, just making things up as you go isn't the best strategy either, because you can easily write yourself into a corner that way.
The result of that uncertainty is that writers of long-running franchises usually have one overarching plot that gets almost no real development (like the search for the "One Piece" treasure in One Piece), but at the same time continuously start and finish several overlapping secondary story arcs that are much shorter and thus easier to plan. That way they can easily keep the franchise running as long as the audience demands it. And should the audience decline to a level where the economic viability of the franchise becomes questionable, they can then wrap up the franchise with a satisfying conclusion by wrapping up all the still ongoing secondary story arcs and the overarching main plot as well.
That means that the writers only need to plan as far ahead as their currently ongoing secondary story-arcs reach while also keeping an emergency plan to quickly wrap up the main plot.