In Captor Family fanfic, the theme of protecting the family's honor is quite prevalent. The family members strive to uphold the good name of the Captor family. There's also the theme of inheritance, not just in terms of material wealth but also in terms of family values and traditions. These themes help to create a rich and engaging narrative that draws readers into the world of the Captor family.
Another possibility is a group of corrupted Plumbers. They turn against Ben and capture him, perhaps because they want to use the Omnitrix for their own evil purposes. They would have knowledge of Ben's abilities and how to contain him, which would make for an interesting conflict.
Maybe some mad scientist who wants to experiment on Beast Boy's shape - shifting DNA. He could be looking for a way to transfer the ability to himself or create some sort of mutant army. Another option is that it's a group of aliens who mistake Beast Boy for a dangerous creature and capture him to study. They might not understand that he's a sentient being with friends who will come looking for him.
It could be Captain Hook. He has always been Peter Pan's nemesis and has a strong motive to capture him. Maybe he wants to finally get revenge for all the times Peter has outsmarted him.
A fanfic could involve a character from Naruto being possessed by an evil card from Sakura Card Captor. For example, if Sasuke were to be possessed by a card that amplifies his hatred. Naruto would then have to team up with Sakura Kinomoto to find a way to free Sasuke. They might need to use a combination of Naruto's chakra control and Sakura Kinomoto's card -capturing abilities.
There's a possibility that it's a villainous trainer. This trainer might capture Gengar with the intention of using it for evil purposes, like using Gengar's powers to steal other Pokémon or cause chaos in Pokémon battles.
Maybe it's an old foe who has managed to regain power and wants to make Sora pay for past defeats. For example, a powerful dark sorcerer who was once defeated by Sora but has now found a way to capture him through some devious plot.
Sadly, without more in - depth research, I can't name the exact captor in the 'Watch Girl in the Basement' true story. But typically, captors like this are people who want to isolate their victims from the world and create a situation where they have absolute authority. They may be people who lack empathy and have a need to control every aspect of someone else's life, which is truly a terrifying and immoral behavior.