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I mean this is a pretty wacky story
People often say that Oda has a crazy imagination.
While he has a good one, the factor behind what you see in One Piece is not imagination but storytelling and a technique called
Milking. I will give you a very simple example without talking about One Piece:
Let's say that you want to make an SF horror story that criticize capitalist and a strong woman. Well, the best thing you could do is to create a slasher in space and transform your threat into an alien.
Now the question is... How would you make this threat scary ? Well.. Simple, make the threat almost invicible and highly scary looking, like a monster from your nightmares.
> Not scary enough ? That's right, make this creature grow in real time and make the people who need to survive stuck into little corridors. Corridors used by the beast to move around. Make it extremily furtive.
> Not scary enough ? Give scary properties to the beast in case your destroy it : A killer acid blood should do the trick.
Now... How do you create a political critic of capitalism ? Well, you could transform your future victim into the double victim of both the beast and a capitalistic move. Best way to do that is to transform your tired crew with a covoyer/truck driver esthetic, with people who eat junk food and smoke all day and talk to eachothers like old bar buds. Now for the critic, let's say that they were tricked into a mission of recuperation and their safety was not taken into account.
> Not enough, well let's say that one of them is a traitor, an android whose mission is to priotize the recuperation mission on top of the safety of the crew.. Even killing them if necessary.
Now.. Your main protagonist must be a strong woman ? How do you do that ? Well, there is a way.. On top of being a scary beast, transform your alien into the representation of a rapists that will stop at nothing to "impregnate" the woman. And make her fight until the very end, even if you have to make her fight in panty if you need to. Make her a badass girl who actually is the smart one.
...
This is what Milking is. And in just by following a premiss "a SF horror story with a strong woman and a capitalist critic", you got Alien. Not much imagination was needed, just a good narrative thinking: