Maybe it's a new villain in the detective - thief world. This villain wants to make a name for themselves by taking on both Kaito and Shinichi at the same time. They might be very cunning and have planned every detail of the kidnapping carefully.
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.
In such fanfiction, Shinichi is often depicted as still having his sharp detective mind. He's intelligent, observant, and tenacious in solving problems. Kaito, on the other hand, is shown as his flamboyant self, with his amazing skills in magic and disguise. He's also quite charming. Both of them might have a bit of a competitive streak towards each other, which adds to the tension and interest in the story.
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.
One possible storyline could be that they are kidnapped by a criminal organization that wants to use their intelligence for some evil plan. For example, they might be forced to solve complex puzzles or decrypt secret codes. Another storyline could involve a rival thief or detective who kidnaps them to prove their superiority. They could be held in an elaborate, hidden location and have to use their wits to escape.
One possible plot twist could be that the kidnapper is actually someone they both know in disguise. Maybe it's a rival detective or magician who wants to prove they are better. Another could be that the place they are kidnapped to has some sort of hidden treasure or secret that the kidnapper is forcing them to find.
Their relationship could develop in many ways. For example, Kaito might become more responsible and caring towards Shinichi during the pregnancy. He could start to see Shinichi in a new light, not just as a rival but as someone he truly loves and wants to protect. Shinichi, on the other hand, might become more reliant on Kaito, which could strengthen their bond.
In Kaito and Shinichi lemon fanfiction, a frequent theme is the idea of forbidden love. Their roles as a phantom thief and a detective respectively create an interesting backdrop for this. There's also often a focus on the physical attraction between them. The fanfictions may explore how they deal with their feelings while still maintaining their public personas. Additionally, some stories might involve themes of redemption or transformation as they navigate their relationship.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.
It could be an orc warband. Orcs have always been enemies of the elves, and they might see Legolas as a valuable captive either for ransom or to torture. Another possibility is a rogue wizard. Wizards who have turned to the dark side often seek powerful beings to experiment on, and Legolas, with his elven magic and skills, would be an interesting subject.
As mentioned before, Eggman is a likely candidate. He's always had his eyes on Tails' skills and would stop at nothing to use them for his evil plans. So he could be the one behind the kidnapping.