Revenge could also be a motive. Maybe Naruto has unknowingly wronged someone or is associated with a group that the kidnappers have a vendetta against. So they kidnap and enslave him as a form of revenge.
One possible motive could be power. If Naruto has some unique abilities or is a key to something powerful in the fanfic world, the kidnappers might want to control him to gain that power for themselves.
One possible motive could be to use Hermione as a bargaining chip against Harry Potter. Since Hermione is his close friend, the kidnappers might think they can force Harry to do something, like give up a powerful magical item or valuable information.
It could be Orochimaru. He has always been interested in Naruto's power and potential. He might kidnap Naruto to try and extract the Nine - Tails' chakra or study his unique Uzumaki bloodline further. Another possibility is a group of rogue ninjas who want to sell Naruto to the highest bidder, thinking his value as a Jinchuriki would make them rich.
The kidnapper could be an estranged family member of Beckett. There might be some family feud or inheritance issue that leads them to believe that by kidnapping the baby, they can gain an upper hand. Or it could be a completely random person who has a mental disorder and randomly targeted Beckett and the baby because they saw them as an easy target.
One possible motive could be ransom. If Himawari is from a well - off family, the kidnappers might be after a large sum of money. Another motive could be revenge against someone close to Himawari, like her parents. Maybe they have some sort of vendetta and think kidnapping her will be a way to get back at them.
Another motive could be related to Kirishima's unique quirk. Maybe the kidnappers are interested in studying or stealing his hardening quirk for their own malicious purposes. They could be part of an illegal quirk - trading organization or some sort of mad scientist's group.
One possible motive could be to gain power. Since Goten is a Saiyan, his power could be harnessed in some way. For example, they might want to extract his ki or use his DNA for some evil experiment.
One possible motive could be to gain power. Natsu is a Dragon Slayer, and his magic is very powerful. The kidnappers might want to extract his magic or use him as a weapon. Another motive could be for revenge. Maybe Natsu has unknowingly wronged someone in the past, and they are out for vengeance. Or perhaps they are working for a greater evil, and Natsu is seen as a threat to their plans, so they want to get rid of him by kidnapping him.
One possible motive could be power. The kidnappers might think that by having Princess Peach, they can control the Mushroom Kingdom. Since she is the ruler, her capture could give the kidnappers leverage to make demands on the kingdom's resources or governance.
He could use his ninja skills. Naruto is a highly skilled ninja. He might find a moment of weakness in his captors' defenses and use his combat abilities, like his Rasengan or Shadow Clone Jutsu, to break free.
It could be a group of rogue shinobi from a neighboring village who are jealous of Konoha's power and see Hinata as a valuable hostage. They might think that by kidnapping her, they can gain some leverage over Konoha in terms of resources or territory.