Well, it could be that Luffy gets kidnapped because of some secret he holds. Maybe there's a group that wants to find out the location of One Piece through Luffy, and they think kidnapping him is the best way. His friends then have to search for clues and race against time to save him before he's forced to reveal the information.
Well, in this scenario, Luffy's crew would surely go all out to rescue him. Nami might come up with a clever plan using her navigation skills and knowledge of the area where Doflamingo is likely to be hiding Luffy. Zoro would be ready for a fight, his swords sharp and his determination unwavering. They might also get some unexpected help from other characters in the One Piece world who have a grudge against Doflamingo.
If Luffy was kidnapped in a fanfiction, his crew would surely go to great lengths to rescue him. Nami would probably come up with a smart plan, using her knowledge of the seas and navigation. Zoro would be ready for a fight, not fearing any opponents in the way. Sanji would keep everyone fed during the rescue mission, while also being prepared to kick some kidnapper butts with his powerful kicks.
One possible plot could be that Aladdin is kidnapped by a powerful sorcerer who wants to use his knowledge of the magic lamp to gain more power. His friends, Jasmine and Genie, then have to embark on a dangerous journey to rescue him. They might face various obstacles along the way, like magical traps and other evil minions.
One possible plot could be that Luigi is kidnapped by a group of mischievous goombas who are being controlled by a powerful dark magic user. Luigi, while kidnapped, uses his wits to befriend some of the weaker goombas and gets them to help him send a message to Mario. Mario then has to figure out where Luigi is being held based on the clues in the message.
Well, it could be about a person, maybe a young adventurer, who is captured by pirates. The story might then revolve around their attempts to escape or adapt to life on the pirate ship.
Well, it could be that pirates kidnap Moana. They have heard tales of her island's treasures. Once on their ship, Moana has to face the rough and rowdy pirates. She manages to befriend some of the more sympathetic ones among them and together they plot against the cruel captain. Eventually, she uses her knowledge of the sea to help them all when they get into trouble, and in return, they set her free.
The plot might involve a character being lured into Pennywise's lair, perhaps by the allure of missing things or people. Pennywise could use his shape - shifting abilities to deceive the kidnapped character throughout their captivity.
In a 'Vegeta kidnapped' fanfiction, perhaps Vegeta could be kidnapped by an old enemy seeking revenge. This enemy might be from his Saiyan past. They could be using him to draw out Goku, thinking Goku will come to save Vegeta. Vegeta, being proud, would likely struggle against his captors and try to break free on his own at first.
One possible plot could be that Levy is kidnapped by a rival guild who wants to use her knowledge of ancient magic texts for their own gain. Her friends in Fairy Tail then embark on a dangerous rescue mission, facing various traps and enemies along the way.
One possible plot could be that they kidnap a character to gain some sort of power. Maybe they think the kidnapped character has a special ability or knowledge that they need. For example, if it's in a superhero - themed fanfiction, the kidnapped character might be able to control an element, and Will and Phobos want to use that power for their own evil plans.