Perhaps it's a villain from the Naruto universe who has been given a different backstory in the fanfiction. They might kidnap Naruto to use him as a bargaining chip against the Hidden Leaf Village. For example, if Orochimaru in this fanfic has some new motivation related to Naruto's power, he could be the kidnapper.
Well, it could be an obsessed fan who wants to keep Naruto all to themselves in the fanfiction. Maybe they think Naruto has some special power they can use for their own gain.
In some fanfictions, it could be a new character created by the author. This new character might be from a different clan or group that has some grudge against Naruto or the village he represents. They see kidnapping Naruto as a way to make a statement or get revenge.
It could be a rogue ninja who wants to sell Naruto to the highest bidder. There are always those in the Naruto world who are after power and money.
It could be Orochimaru. He is always on the lookout for powerful specimens to experiment on or train in his ways. Naruto, as the son of the Fourth Hokage, would be an extremely appealing target for him at birth.
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 also be a secret organization within the Naruto universe that we don't know much about in the original story. This organization might be aware of some hidden prophecy or potential in Naruto related to the Nine - Tailed Fox. They kidnap him with the intention of either preventing that potential from being realized (if they see it as a threat) or using it for their own mysterious ends.
One possibility is that Naruto's friends launch a rescue mission. They would use their unique skills, like Sasuke's Sharingan or Sakura's medical ninjutsu combined with her combat skills, to find and save Naruto.
It could be Drakken. He's always trying to defeat Kim and might think kidnapping her is the ultimate way to get the upper hand. He could be planning to ransom her back to her family or use her to carry out his evil plans.
Some writers might do it to create a more complex and multi - faceted Naruto. By putting him through such a harrowing experience, they can show different aspects of his personality, like his determination, his loyalty to his friends even in the face of great pain, and his ability to rise above the situation. Also, it could be a way to introduce new and original plotlines that deviate from the main story but still stay within the Naruto world.
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.
One possible motive could be revenge. Maybe Reid has put away a relative or someone close to the kidnapper in the past. Another motive might be related to Reid's intelligence. The kidnapper could want to use his brainpower for some illegal or self - serving scheme, like cracking a code or coming up with a complex plan.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.