In some fanfictions, it could be a mysterious figure who has been observing Raven for a long time. This person might be interested in her unique abilities and wants to study or control her for their own gain.
Enemies from her past, such as old villains she's fought against, would be likely kidnappers. They might hold a grudge for her previous defeats and want to use her for revenge or some dark purpose.
Maybe a secret admirer who is too shy to approach her normally. They think that by kidnapping her, they can have some alone time with her and confess their feelings. However, this is a very extreme and wrong way to go about it. Another possibility is a group of thieves who think Haruhi comes from a wealthy family and they can get a big ransom for her.
The Yiga Clan could be likely kidnappers. They've always been enemies of Link and would love to get their hands on him. They might see kidnapping him as a way to stop him from interfering in their plans. Another possibility is the Gerudo thieves. Although not all Gerudo are bad, there could be a group of them who think Link has something valuable and decide to take him hostage.
It could be a rival team's coach or a jealous player. They might see Kuroko as a threat to their success and decide to kidnap her to keep her out of important games. For example, if there's a big championship coming up and they know Kuroko's skills are crucial for her team's victory.
It could be a jealous rival host club. They might be envious of the kidnapped host club's success and popularity, so they plan to kidnap them to gain an advantage.
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 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.
A secret society that believes Mycroft is a threat to their hidden agendas. They may think that by kidnapping him, they can stop him from interfering with their plans. They would likely be very well - organized and have resources to keep him hidden.
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.