The Grimm could also be involved in an unusual way. Perhaps a new type of Grimm that has the intelligence to kidnap. They might be working with a human or Faunus villain who has some sort of control over them, and they take the character to an unknown location where the RWBY team has to face not only the Grimm but also the mastermind behind it.
The Joker is a likely kidnapper. He has a history of causing chaos and targeting the Bat - family. He might see kidnapping Batgirl as a way to get back at Batman or just for his own twisted amusement.
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.
Perhaps a rival community. They might feel threatened by Carl's group and think that by kidnapping Carl, they can gain an advantage. They could be more organized and have a specific plan in mind, like using Carl to get access to their resources. It could also be a group of escaped prisoners. They have no moral compass and figure that kidnapping Carl can help them get whatever they want in this lawless world.
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.
It could be a group of rogue ninjas. They might see Shikamaru as a valuable asset due to his intelligence and strategic skills. They could plan to sell him to the highest bidder or use him to carry out their own devious schemes.
It could be a rival athlete who is jealous of Serena's success. They might think that by kidnapping her, they can eliminate the competition. Another possibility is a deranged fan who has an unhealthy obsession with Serena and wants to keep her all to themselves.
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.
One possible kidnapper could be a rival engineer who is jealous of MacGyver's fame and ability to solve problems so effortlessly. This person might think that by kidnapping MacGyver, he can steal his ideas and take credit for his inventions. Also, it could be a group of eco - terrorists who believe MacGyver's work has been harmful to the environment in some way and want to force him to make amends by using his skills for their cause. A third option is a deranged fan who has an unhealthy obsession with MacGyver and wants to keep him all to himself, thinking that kidnapping is the only way to have MacGyver's undivided attention.
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.