Well damn, apparently I need to read more One Piece.
Speaking of One Piece. I had an idea that I might post snippets of here
It has two directions, one of the directions Taylor, Deku, Ruby, and my own character Ultimate end up in One Piece. Ultimate makes a ship and they sail to find their way back home.
The other directions is I could make it a Fusion Fic. Where Captain Izuku works with His first mate, Taylor, his sniper, Ruby, and traitorous shipwright, Joey.
I'm going to give the inverse of Obloquy's advice and say that you should start by figuring out why each character ends up as a pirate. It's easier in the non-fusion-fic, because they have a clear goal that could clash with the World Government, and in any case each of them has morals that clash with the Government's justice. The hard part might be figuring out how to justify a de-iseka-inator that the quarter could track down existing on the Grand Line.
With the fusion fic, you'd need to reverse engineer their backstories in a way that gives them sufficient conflict with the World Government that they end up pirating together. I don't know much about Ultimate, and what little I do know makes it seem like he'd end up in conflict with the WG anyhow, so I'll focus on the other three.
Taylor is honestly pretty easy, because her backstory can easily be transposed into pretty much any universe with functional institutions
and comes with a free reason to dislike the Government. The biggest risk would be One-Piece-ifying it in ways that make her story worse, like making Emma and Sophia World Nobles who Taylor somehow escaped from, or copying 90% of Robin's backstory.
She's a borderline outsider of some kind, who ends up getting pushed definitively into the "okay to mistreat" zone by people with a bit more respectability than her. Eventually, she happens to find the Swarm-Swarm Fruit and gets the hell out of Dodge Island. Easy.
Ruby and Izuku are a bit trickier, because their backstories align them strongly with the Powers That Be.
Ruby wants to be a Huntress, both because of the variety of great heroes who have been Huntsmen and Huntresses in the past, and because her parents and their social circle were all also Huntsmen. For all that Huntsmen help people, they also protect the status quo, inequities and all.
The easiest way to square this circle would be to find-replace "Huntress" with "Revolutionary". They're also a community of badasses who mostly know others from that community. But that leaves you with the problem of why she would end up on some random pirate ship.
Maybe Ruby's family could be from some local warrior order that comes into conflict with the World Government? Something that the World Government could disrupt or scatter without completely fucking over some other aspect of the setting. I'm imagining something like the Kuja Pirates, but with less plot relevance.
And then there's Izuku. Sweet summer child Izuku. His backstory is built around two conceits: He is an ordinary human in a world of superhumans, and he idolizes the man who represents the world's most important rotten institution.
I don't know if you can imitate Izuku's Quirkless baggage in
One Piece's setting. But I have a pretty good idea of how you could wrangle the other.
First off, Izuku's idol is Monkey D. Garp, Hero of the Marines. Second, he's a young seaman in the Marines who is recognized as having potential. Maybe Garp trained him personally when he was a mere chore boy, since that's the closest he can come to passing on the Quirk he doesn't have.
Second, Izuku's ship ends up on the Marine ship that gets sent after the other three characters once they've attained a drop of notoriety. He's given chances to capture or kill them at some cost his personal sense of justice is unwilling to pay, and his "weakness of character" sees him punished by his superiors. Optionally, rinse and repeat for a couple confrontations.
Eventually, the Marines call him a traitor and throw him overboard. Luckily, the pirates are willing and able to fish him out of the sea before he drowns.
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