Well, she could be put on public display. The Capitol loves spectacle. They might dress her up in elaborate gowns, like they did before the Games, but this time to show their power over her. Her captors might also try to extract information about the rebel plans from her. However, Katniss is a tough character, so she would most likely resist as much as she can, even in captivity.
If Katniss is captured by the Capitol in fanfiction, it could lead to a whole new set of challenges for the rebels. Her capture would be a huge blow to their morale. The Capitol might attempt to brainwash her, make her forget her loyalty to the rebels. But Katniss has a strong will. There could be a sub - plot where some of the Capitol citizens start to have doubts about their own regime after seeing Katniss in captivity, as she represents the spirit of resistance.
In this fanfic, Katniss might face intense torture. The Capitol is known for its cruelty. They could use various methods to break her spirit, like starving her or subjecting her to mind - games. Maybe they'll try to use her as a tool to control the rebels, threatening to kill her if the rebellion doesn't stop.
One way could be through alliances. In the Capitol, there might be some people who secretly support her cause or who are simply against the cruel treatment of captives. She could form an alliance with them and plan an escape. For example, a Capitol servant who has access to keys or security codes could help her. Also, Katniss could pretend to be compliant at first, lulling her captors into a false sense of security before making her move.
Peeta could be the one to save her. He has always cared deeply for Katniss. He might gather a group of rebels secretly and launch a rescue mission. Maybe he'll use his charm and skills to infiltrate the Capitol and find where Katniss is being held.
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, 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, 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.