It could be President Snow's loyalists who are trying to make a last - ditch effort to undermine the rebellion. They might think that by kidnapping Katniss and Peeta, they can demoralize the rebels. After all, Katniss and Peeta are symbols of the rebellion.
There's also the possibility that it's a secret organization within the Capitol that has its own agenda. This organization could be made up of people who were against Snow but also don't want the rebels to completely take over. So they kidnap Katniss and Peeta to try and negotiate a new order where they have a say.
It could be a rival spy agency. They might see Moneypenny as a valuable target to get information about MI6's operations. They would likely be highly trained and well - equipped.
Maybe it's a character from one of the games Matpat has theorized about. For example, if he made a theory about a game villain being more complex than thought, that villain could come to life in the fanfiction and kidnap him to prove his own power and intelligence.
The kidnapper might be a deranged Toad. Maybe this Toad has been corrupted by some evil force and decides to kidnap Luigi for reasons only known to the twisted mind of this new 'evil Toad'.
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.
The kidnapper could be a former ally who has turned against Luffy. Maybe they've been corrupted by power or some other force. They believe that by kidnapping Luffy, they can take over the Straw Hat Pirates or get their hands on valuable resources that Luffy has access to.
It could be Frieza. Frieza has a long - standing grudge against the Saiyans, especially Vegeta. He might kidnap Vegeta to torture him or to use him as a bargaining chip against the other Saiyans. Another possibility is Cell. Cell might see value in Vegeta's genetic material and kidnap him to extract it for his own evolution.
It could be an enemy from one of his past cases. Someone who holds a grudge against L for getting them caught or ruining their plans before.
There's a chance it could be a character with a hidden agenda within her own circle. Maybe someone who seems friendly on the surface but has ulterior motives. They kidnap her to further their own goals, which could be related to power or money within their community.
It could be a jealous rival player from another school. This player might feel that Hinata is getting too much attention and success in volleyball, so they decide to kidnap him to stop his progress.
It could be a rival guild member who wants to use Juvia as leverage against Fairy Tail. Maybe someone jealous of her powers or her relationship with Gray.