It could be Team Rocket. They are always up to no good and might see Diamond as a valuable asset to either sell or use for their evil plans.
Another possibility is that a jealous rival trainer kidnaps Diamond. This rival is envious of Diamond's skills and success and hopes to gain an edge by holding him hostage, perhaps to get access to his Pokemon team or training secrets.
Maybe a new, mysterious villain who has some grudge against Diamond or the Pokemon world in general. This villain could be after Diamond's power or knowledge about certain Pokemon evolutions.
Well, it could be that a powerful villain kidnaps Diamond to use as a bargaining chip against the Pokemon trainers. Maybe the villain wants to steal rare Pokemon and believes Diamond has the knowledge to help them find these Pokemon. So, Diamond is held captive in a secret lair filled with all sorts of traps and guards.
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.