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.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.
In some fanfictions, when Gengar gets captured by Ash, it might lead to some really interesting team dynamics. Ash could use Gengar's unique abilities in battles. For example, Gengar's ability to phase through walls could be used in creative ways during gym battles or against Team Rocket. Maybe Gengar would initially be a bit rebellious, but over time, it would grow to respect Ash as its trainer. Ash would also have to find new ways to train Gengar compared to his other Pokémon, since Gengar has different strengths and weaknesses.
Well, in the fanfiction, Gengar might be held captive by some powerful villains who want to use its powers for their own evil plans. Maybe they'll try to force Gengar to haunt a particular place or to attack someone. The Pokémon trainers in the story could then start a rescue mission to save Gengar.
The story could develop in many ways. For one, Ash might enter new tournaments with Gengar by his side. Gengar's presence could attract the attention of other trainers who are either scared or intrigued. This could lead to some unique battles where Ash has to prove that he can handle Gengar well. Also, Gengar might have some secrets or a past that gets revealed as it spends more time with Ash. Maybe it was once part of an evil team but now has a chance to be good with Ash as its trainer.
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.
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, 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.