A jealous colleague from another department might be the kidnapper. Maybe they feel overshadowed by Reid's achievements and decide to take him out of the picture. They could have planned the kidnapping meticulously, using their knowledge of the BAU's routines to their advantage. They would want to make it look like a professional criminal's work to mislead the investigation.
Perhaps it's someone related to a victim that Reid couldn't save. This person blames Reid for the loss and in their grief - turned - rage, decides to kidnap him. They might be thinking that by making Reid suffer, they can somehow get justice for their lost loved one. This type of kidnapper would be more emotional and less organized than a typical criminal, which could lead to different challenges for Reid during the kidnapping situation.
It could be a copycat unsub who idolizes the previous criminals the BAU has caught. He might have studied Reid's work and decided to target him to make a name for himself in the criminal world.
Maybe it's a cult leader. The cult might believe that Reid has some sort of special knowledge or power that they want to harness. They kidnap him to convert him to their beliefs or use his mind for their own strange purposes.
It could be a deranged fan of one of Reid's academic works. This person might have misinterpreted Reid's theories and decided that Reid was some sort of false prophet. So, in his twisted mind, he kidnaps Reid to 'correct' him.
One possible plot twist could be that the kidnapper is someone from Reid's past that he thought was long gone. Maybe an old unsub who escaped prison and is seeking revenge. Another could be that Reid is kidnapped as a means to get to the BAU team, and the kidnapper is using him as bait to draw out the whole team for a bigger attack.
It could be the Joker. He's always up to no good and kidnapping Robin would be a great way to mess with Batman. The Joker might want to use Robin to force Batman into a series of cruel and twisted games.
Maybe an enemy from another village. If there are political tensions between villages, kidnapping Sarada could be a way to gain an advantage. They might think that by having her, they can force her village to make concessions in treaties or resources.
It could be a new villain who has heard about Tails' mechanical skills. This new baddie might think that by kidnapping Tails, they can force him to build powerful weapons for them.
It could be a rival magic knight. Maybe someone from a different squad who is jealous of Asta's rapid rise in the ranks and the attention he gets. They might think that by kidnapping him, they can make themselves look better or get rid of the competition.
Maybe a human who has discovered the existence of vampires and wants to use Renesmee to prove it to the world. They might think that by kidnapping her, they can expose the vampire world and gain fame or fortune. Another option is a rogue werewolf who doesn't like the alliance between the vampires and werewolves and decides to take Renesmee as a way to break that bond.
One possibility is Dr. Eggman. He has always been after Sonic and wants to stop him. Kidnapping Sonic would be a way for him to achieve his evil goals.
A new villainous trainer group. They might see Ash as a threat to their plans of dominating the Pokémon world, so they kidnap him to stop him from competing in important battles or tournaments.