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 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 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.
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'.
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.
Maybe it's a group of poachers who are after rare Pokémon. Gengar, being a relatively unique and powerful ghost - type, would be a valuable catch for them. They could plan to sell Gengar on the black market for a high price.
There are several candidates. It could be a rogue demigod who has turned to the dark side. They might think that by kidnapping and torturing Annabeth, they can gain power or recognition. It could also be a minion of a major god who is acting on their master's orders, perhaps because Annabeth has somehow angered that god. Additionally, it could be a mysterious figure from a long - forgotten prophecy who believes that Annabeth is the key to fulfilling their dark desires.
It could be another werewolf from a different pack. There are often rivalries between packs in the Teen Wolf universe, and someone might see Liam as a way to gain an advantage over his pack.
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.