A new villain, a dark ice - wizard from a far - off land, could be the kidnapper. He might have heard about Anna's connection to Elsa and her powers and wants to use Anna to control Elsa. He kidnaps Anna and takes her to his frozen lair.
Maybe a group of anti - magic fanatics kidnap Anna. They believe that Elsa's magic is a threat and they think that by kidnapping Anna, they can force Elsa to leave the kingdom and never use her magic again.
Well, it could be that Anna is kidnapped by an evil sorcerer who wants to use her as a bargaining chip to get Elsa to give up her powers. Maybe he locks her in a far - off tower and Elsa has to embark on a dangerous journey to rescue her sister.
It could be Death Eaters who are still lurking in the shadows. They might see Hermione as a threat because of her association with Harry Potter and her strong stance against the Dark Arts. They could kidnap her to try and get information about Harry or to use her as bait.
It could be the well - known Team Rocket. They are always up to no good and kidnapping Pokemon for their own gain is right up their alley.
It could be a jealous rival from his past. Maybe someone he outperformed in a competition or a former acquaintance who has a grudge against him.
Well, it could be a secret organization that has been operating in the shadows. They might have some ulterior motive related to Naruto's power or his connection to the Nine - Tails. Maybe they believe that by kidnapping Naruto, they can control the Nine - Tails and use it for their own purposes. Another option is a jealous ninja from within Konoha. This ninja might feel overshadowed by Naruto's achievements and popularity and decides to kidnap him to get him out of the picture. Also, a powerful and ancient being awakened from a long slumber could be the kidnapper. They see Naruto as a source of energy or a key to something they desire.
It could be Vilgax. He has always been after the Omnitrix and kidnapping Ben 10 would be a way for him to get it.
Hans could be a likely villain in these fanfictions. Since he already had evil intentions in the movie, it's easy to imagine him coming back to kidnap Anna for revenge or to try and take over Arendelle again. Another one could be a power - hungry sorcerer from a far - off land who wants to use Anna as a bargaining chip against Elsa.
Perhaps a wizard from a foreign magical school who is jealous of Hogwarts' reputation and believes that by kidnapping Hermione, he can undermine Hogwarts. He may think that her abduction will cause chaos and damage the school's standing in the wizarding world. Also, it could be a magical being that has been corrupted by dark magic and views Hermione as a threat to its new - found power, so it takes her away.
It could be a rogue ninja who wants to use Boruto to get to Naruto. Maybe someone who has a grudge against the Hokage.
It could be Orochimaru. He's always been interested in Sasuke and might see kidnapping Sakura as a way to manipulate Sasuke. He could use Sakura as a guinea pig for his experiments or as a bargaining chip to get Sasuke to come to him.