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.
Orcs would be a likely kidnapper. They have a long - standing enmity with the elves, and Legolas being an important elf, would be a valuable captive. They could either want to torture him for information or use him as a bargaining chip with the elves.
It could be an obsessed fan. You know, someone who has taken their admiration too far. They might think that by kidnapping Liv and Maddie, they can have them all to themselves and be part of their exclusive world. This type of kidnapper would be really unpredictable as they are acting out of an extreme and abnormal form of love.
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 someone Reid put away in the past. Maybe a criminal who has held a grudge against him ever since and has now found an opportunity to kidnap him for revenge.
It could be a jealous rival. Maybe someone who is envious of Felicity's intelligence or her relationships. Without her glasses, this person might think she's more vulnerable and easier to manipulate or hold hostage to get what they want.
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 be a jealous coworker. Maybe they feel that Kaoru is getting all the attention or a promotion they wanted. Another possibility is a patient who has become obsessed with Kaoru during their treatment. Or perhaps it's someone from Kaoru's past who has a grudge against her.
The Black Hood is a likely candidate. Since he has a history of causing trouble in Riverdale, he might kidnap someone to send a message or continue his reign of terror. He could target Cheryl, for example, thinking she's a symbol of the wealthy and corrupt in the town.
A powerful and insane magical being from a far - off land. For example, a powerful sorcerer from a place like the Forbidden Forest that has long been in hiding. He hears of the prophecy about Harry Potter and decides to kidnap him as a baby to study his magic and use it to gain more control over his own territory.