Perhaps in this fanfiction, Sora gets captured because he was lured into a trap. His captors could be using him as bait to draw out other important characters. While captive, Sora might try to escape on his own by using his wits and any small powers he can muster in the confined space. His friends, on the other hand, are frantically searching for clues to his whereabouts.
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, 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, 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 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, 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.
Another scenario could be that the kidnappers have some sort of ulterior motive related to Sora's special powers. They might try to extract that power from him, but Sora, being the strong - willed character he is, would fight back in some way, even while being held captive.
Maybe it's an old foe who has managed to regain power and wants to make Sora pay for past defeats. For example, a powerful dark sorcerer who was once defeated by Sora but has now found a way to capture him through some devious plot.
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.
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.
In a fanfic where Arya gets captured, she might be used as a bargaining chip. Her captors could try to exchange her for something valuable, like a strategic location or a large sum of gold. Maybe they want to use her to manipulate the Starks.