It could also be a lone vampire with a vendetta against Kaname or the academy. This vampire might think that by kidnapping Yuki, they can exact revenge on Kaname or cause chaos within the academy. Yuki is an important figure in the Vampire Knight world, and her kidnapping would surely disrupt the delicate balance that exists.
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.
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.
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.
One likely kidnapper could be Discord. He loves chaos and might see kidnapping Twilight as a great way to create chaos in Equestria. He could be bored and think that Twilight's absence would disrupt the harmony that she and her friends usually maintain. Another possible kidnapper is a jealous rival magician. Someone who is envious of Twilight's power and knowledge of magic and believes that by kidnapping her, they can steal her secrets and become the most powerful magic user in Equestria.
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.
Perhaps Kaname would discover her disappearance and go on a quest to find her. Given his deep connection to Yuki, he would stop at nothing to get her back. He could use his pureblood vampire powers to intimidate those who kidnapped her into revealing her location.
There could also be authors who are more interested in creating drama and conflict. Bashing Yuki can create a lot of drama within the story, as she is a central character in 'Vampire Knight'. So, they use her as a tool to generate more interesting and controversial storylines.
There could also be a group of villains working together to kidnap Sonic. For instance, a combination of Eggman and some of his more powerful robot creations along with a couple of other villains from Sonic's rogues gallery. They might have a grand plan that requires Sonic's capture, like using his power to open a portal to a world full of chaos energy that they can then control.
A jealous rival of Inuyasha might kidnap Kagome. For example, another demon who wants Inuyasha for himself might think that by taking Kagome, he can get Inuyasha to do his bidding. This would create a lot of drama and conflict between Inuyasha and the kidnapper.
It could be a vengeful god or goddess. Maybe one who has a grudge against the demigods in general and sees Annabeth as an easy target to send a message. For example, a minor god who feels overlooked and wants to prove their power by kidnapping such a well - known demigod.