One potential kidnapper could be a dark wizard who wants to use Lucy's Celestial Spirit keys for their own dark magic. Another could be a rival guild member who is jealous of Lucy's power and wants to steal her keys to gain an advantage in the magical world. It could also be someone from Lucy's past, like an old acquaintance who has turned bad and has some grudge against her.
A group of rogue Celestial Spirit users could be the kidnappers. They might believe that Lucy is misusing her powers and they want to take the keys away from her to use them 'properly'. Another potential kidnapper could be a demon who has been sent from the underworld. The demon might have been ordered to capture Lucy as part of a plan to start a war between the Celestial and underworld realms. And there's always the chance that it's a power - hungry politician in the magical world who thinks that having control over Lucy's powers will give them more influence.
Well, perhaps it could be a cult that worships some dark power and believes that Lucy, with her connection to the Celestial Spirits, is the key to unlocking a great source of evil magic. Or it might be a bounty hunter who has been hired by someone who wants to collect the reward for capturing a Fairy Tail mage. There's also the possibility of a mad scientist - type character who wants to study Lucy's unique magical abilities and her connection to the spirits.
The Akatsuki could be potential kidnappers. They are always on the lookout for powerful jinchuriki like Naruto to extract the tailed beasts from them. Orochimaru could also be a kidnapper as he is always interested in obtaining powerful bodies and Naruto's strength would be very appealing to him.
A new, unknown criminal organization could be behind the kidnapping. They might have heard about Duke's potential and see him as a valuable asset. They could be planning to either brainwash Duke to join them or use his abilities to carry out their illegal activities.
Some disgruntled members of the Sand Village itself could be the kidnappers. Maybe they feel that Temari or her family has wronged them in some way. They could kidnap her in an act of revenge. This would add an interesting layer of complexity to the story as it would involve internal village politics and betrayal.
It could be the Separatists. They might see Anakin as a valuable hostage to use against the Jedi and the Republic. His skills as a Jedi Knight make him a prime target for them to gain some sort of leverage in the war.
It could be a rival family who wants to use Tsuna as a bargaining chip against his own family.
It could be some rogue Hollows who see Ichigo as a big threat and think that by kidnapping him, they can eliminate that threat.
The kidnappers could be a group of hired mercenaries. Some powerful and wealthy person who has something to hide might have hired them. This person could be involved in illegal activities and Castle was on the verge of uncovering the truth, so they order the mercenaries to take Castle out of the picture.
It could be a rival gang. They might see Ponyboy as a threat or a way to get back at his own gang. Another possibility is a deranged fan of his, someone who has an unhealthy obsession and wants to keep him all to themselves. Or it could be a group of criminals who mistake Ponyboy for someone else who has something they want.
The potential kidnappers could be a secret society within the wizarding world that has its own agenda. They might believe that Hermione has a special power or connection that could either help or harm their cause, so they took her at birth to control that. It could also be a non - human magical being, like a centaur or a goblin, who thought that Hermione's birth was a sign of something significant in the magical world. They kidnapped her to study her or use her in their own cultural or magical practices. Or perhaps it was a wizard who had a grudge against Hermione's parents and took their child as revenge.
It could be a rogue vampire who wants Bella for her unique blood. There are many vampires in the Twilight world who might be tempted by her smell. Another potential kidnapper could be a vengeful werewolf who blames Bella for something, perhaps for the way the vampires and werewolves' relationship has changed. It might also be a human who has somehow found out about the vampire world and thinks they can use Bella to gain power or knowledge.