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.
The kidnapper could be a former ally who has turned against Luffy. Maybe they've been corrupted by power or some other force. They believe that by kidnapping Luffy, they can take over the Straw Hat Pirates or get their hands on valuable resources that Luffy has access to.
In the first episode of Heart of Gold in One Piece, Luffy was held hostage by a 200-year-old girl when they arrived at the island. However, because Luffy was the strongest person on the ship, the others were not worried. This was a relatively easy interlude. In addition, there was a similar plot in the One Piece special chapter. There was a little girl named Olga who escaped from the pursuit of the World Government and the treasure hunters and escaped to the spaceship. She was tired and hungry, so she kidnapped Luffy and threatened the other members of the straw hat group to hand over food.
Once Luffy is kidnapped, Doflamingo might try to break Luffy's spirit. But Luffy being Luffy, he would resist. Meanwhile, the crew's journey to rescue him could be filled with many obstacles. They might encounter other pirates or marines who are either allies or enemies of Doflamingo. The crew would have to fight their way through and use their combined skills to reach Luffy. For example, Brook could use his soul powers to pass through walls and scout ahead.
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.
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.
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.
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.
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 Shikadai is kidnapped by a group of rogue ninjas who want to use his intelligence for their own evil plans. His friends then embark on a mission to rescue him, facing various challenges along the way.
Well, it could be that the Flash, perhaps under some mind - control or in a desperate situation, kidnaps someone important in the fanfiction world. Maybe he's trying to prevent a greater evil and thinks this is the only way.