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.
It could be Hiram Lodge. He has his own motives and has been shown to be quite devious. Maybe he kidnaps Archie to manipulate Veronica or to gain some sort of leverage over the town. He might think that by having Archie in his control, he can get the other teenagers to do his bidding.
Maybe it's a deranged fan of one of the Core Four. Somebody who has an unhealthy obsession and thinks that by kidnapping them, they can have a special relationship with their idol. It could be a fan of Archie's football career or Veronica's glamorous lifestyle. They might be living in their own deluded world and believe that the Core Four will come to love them if they keep them hostage.
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.
In a fanfiction, it could be a shadowy organization. They might have heard rumors about Raven's unique powers and decided to take her. This organization could be researching ways to gain superhuman abilities and believe Raven holds the key. Or it could be a deranged fan of Raven's powers. He might be obsessed with her and think that by kidnapping her, he can have her all to himself. Additionally, a vengeful spirit from Raven's past could be the kidnapper, seeking revenge for something she did long ago.
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 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.
Maybe Misa. She's completely devoted to Light, but what if she discovers that Light has been using her all along? In a fit of rage and heartbreak, she could kidnap him. She has the means as she's also involved with the Death Note. She might want to make Light pay for his betrayal or force him to love her truly.
There could be a new dark force in the wizarding world that kidnaps Albus as a way to gain power. They believe that Albus has some sort of latent magic or knowledge that can be harnessed for their evil purposes. Maybe they have some prophetic vision that led them to target Albus specifically.
It could be a jealous rival archer who wants to prove himself better than Roy. This rival might feel that by kidnapping Roy, he can steal his thunder and show the world that he is the superior archer.
A rival mermaid from another tribe could be the kidnapper. Maybe this mermaid feels that Melody is getting too much attention and is seen as a threat to her own status in the ocean community. So, she decides to kidnap Melody to make her disappear.
It could be a vengeful demigod. Maybe someone who lost in a competition to Annabeth and is now seeking revenge.