Well, if Luffy gets captured in the fanfiction, it could lead to some interesting character development. His captors might try to break his spirit, but Luffy being Luffy, he'd probably just keep trying to escape and befriend some of the other prisoners or guards. This could create some unexpected alliances. And his crew would be frantically searching for him, leading to a lot of action and adventure as they follow the clues.
One possibility in the 'luffy gets captured fanfiction' is that his captors are part of a secret organization. They could be studying Luffy's abilities, especially his rubber body and his Haki. Luffy would be constantly trying to break free and find a way to signal his crew. His friends would be using all their skills and resources to track him down. There might be new characters introduced in the process, like other prisoners who have their own stories and might help Luffy escape. It could also lead to a big showdown between Luffy's crew and the captors, with lots of epic battles and the display of powerful abilities.
In this fanfiction, Luffy might be used as a bargaining chip by his captors. Maybe they'll try to exchange him for something valuable, like a powerful Devil Fruit or a large amount of treasure. His crew would then surely launch a rescue mission, facing many challenges along the way.
In a 'naruto gets captured' fanfiction, he might be tortured for information. His captors could be after the secrets of his Nine - Tails chakra. They may try to use him as a bargaining chip against Konoha.
Well, in this fanfiction, perhaps Steven could be held captive by an old enemy. Maybe he'll have to use his wits to escape, like finding a weakness in the prison's security system.
Well, perhaps Gengar is captured by a powerful trainer. The trainer might use a special Poké Ball to catch it. Then, Gengar could try to escape from the ball by using its ghostly powers.
In this fanfiction, perhaps Serena is captured by an evil organization. They might be after her special powers or knowledge. Then, her friends would start a rescue mission.
Well, in such a fanfiction, Sora might be held in a dark and gloomy dungeon. Maybe some old enemies from the original story could be behind the capture, seeking revenge.
Well, he could be held by a villain group who wants to find out the secrets of his power. Maybe they'll try to study his One For All quirk.
Well, if Naruto is captured by Orochimaru in fanfiction, one possibility is that Orochimaru wants to use Naruto as a bargaining chip against Konoha. He may think that by having Naruto in his grasp, he can force the Hidden Leaf to meet his demands, like giving him access to certain forbidden jutsu or secret scrolls. Another aspect could be Orochimaru's curiosity about Naruto's ability to master new jutsu so quickly. He might want to study Naruto's mind and body to find out the secret behind his growth as a ninja.
He might be interrogated by the captors. They could try to extract information about the Sith, the Empire's secrets, or his own powers. Maybe they would attempt to turn him against the Emperor, using various means like appealing to his remaining humanity or threatening him with things he values.
It could be the Marines. They've been after Luffy for a long time, so it's possible they finally manage to capture him in this fanfiction.
In this fanfic, Naruto might be tortured for information about the Hidden Leaf. The Akatsuki are known for their ruthless ways, so they could use various jutsu to make him talk.