A young journalist who has been writing articles about Kaito Kid and his heists. Kaito Kid kidnaps the journalist to control the narrative. He wants to make sure that the journalist doesn't publish any more information that could give away his next moves or expose his secrets.
Perhaps Kaito Kid kidnaps a fan by mistake. He was aiming for someone else who looked similar, and this fan is just an ordinary person who happened to be in the wrong place at the wrong time. The fan could be really scared at first, but then as they interact with Kaito Kid, they start to see a different side of him. Kaito Kid might then have to figure out how to let the fan go without getting caught himself.
It could be a young and innocent bystander. Maybe someone who was just in the wrong place at the wrong time. For instance, a small child who happened to be near where Will and Phobos were planning their kidnapping and was grabbed instead of their intended target.
It could be Iris. Since she's an important character in The Flash's story. The Flash might be forced to kidnap her for some strange reason, like to protect her from a threat that he can't explain to her directly.
If Kaito Kid kidnapped Conan in a fanfiction, it could lead to a very interesting dynamic. Kaito Kid may have kidnapped Conan thinking he's just an ordinary smart boy. But as he realizes Conan's true capabilities, he might have second thoughts. Conan, on the other hand, would use his detective skills to study Kaito Kid's motives and methods. He could also try to communicate with Kaito Kid, appealing to his sense of justice or curiosity to let him go.
It could be a secret society that has been observing their actions and sees them as a threat to their hidden agendas. This society might have members with special skills and resources to carry out the kidnapping.
The victim could be Alec Lightwood. Since Magnus and Alec have a complex relationship, a fanfiction could explore what would happen if Magnus kidnapped Alec, perhaps out of some miscommunication or a threat to Alec's safety that Magnus thought he could only handle by taking Alec away. Another potential victim could be a young, inexperienced warlock. Magnus might kidnap the warlock to teach them about their powers in a more secluded environment away from the prying eyes of the Shadowhunters.
It could be a criminal mastermind who sees Shinichi as a threat to his illegal activities and Kaito as someone who can be used as leverage. Maybe a powerful underworld figure who has been on Shinichi's radar for a long time and decides to take action by kidnapping them both.
Conan is a very resourceful character. He would start by observing his surroundings carefully. If he's in a building, he'd look for exits, vents, or any possible escape routes. He'd also try to communicate with Kaito Kid to gain some insights into his intentions. Conan might pretend to be sick or injured to make Kaito Kid lower his guard. Then, using his knowledge of self - defense and his quick thinking, he would make his move at the most opportune moment. He could also try to use any gadgets he has on him or find something in the kidnapping location to help him escape.
A young boy who is the chosen one to bring about a new era of warmth could be the victim. Jack Frost kidnaps him to prevent the change in the world's climate that would come with the boy's power.
Sherlock Holmes is a great choice. His deductive skills would contrast well with Kaito Kid's thieving skills. Another good one is Lupin III. They are both master thieves, and a crossover between them could be full of interesting heist scenarios. Conan Edogawa would also be interesting. Since he's always trying to catch Kaito Kid in the original series, a crossover could explore new dynamics between them.