A kitchen maid could be the most unexpected rescuer. She might overhear the kidnapper's plans while serving food in the castle. She's not someone you'd expect to be heroic, but she could use her knowledge of the castle's nooks and crannies to outwit the kidnapper and save the daughter.
Perhaps an outcast wizard who has been living in the woods near the castle. He has no love for the castle's inhabitants usually, but for some reason, he has a soft spot for the daughter. He uses his magic, which is different from the normal magic known in the area, to track down the kidnappers and rescue the daughter. His presence is unexpected because he has always been shunned by the castle folk, yet he emerges as the hero.
It could be the castle's old gardener. He has been around the castle for years, knows all the secret paths in the garden and the surrounding areas. He might seem like a frail old man, but his love for the daughter and his knowledge of the land make him a formidable rescuer. He could set traps for the kidnappers using his gardening tools and lead the daughter to safety.
Kagami could also be a likely rescuer. He and Kuroko have a special partnership on the basketball court. Kagami would be furious if someone kidnapped Kuroko and would use his strength and determination to find and rescue him. He might track down the kidnappers through his street - smart skills and his knowledge of the areas where Kuroko could have been taken.
Yoshi could also be the rescuer. Yoshi has his long tongue that can be used to grab things from a distance. He could use it to pull Luigi out of a locked cage or something. And Yoshi is very fast and can jump high too. So he'd be a great candidate to go on a rescue mission for Luigi.
It could also be a new character that is introduced in the fanfiction. Maybe a young girl who has her own set of powers related to light. She discovers Jamie's kidnapping and decides to help. She uses her light powers to dispel the darkness around the place where Jamie is held, blinding the kidnappers and allowing her to free Jamie and lead him to safety, and later on, she might even become an ally to the Guardians.
It could be a rival family. They might be jealous of the Castle family's status or wealth and decide to kidnap them to gain an upper hand. For example, if the Castle family owns a large business empire, the rival family could kidnap them to take over the business.
First, establish the setting of the castle clearly. Describe its grandeur or its dark and mysterious corners. For the daughter being kidnapped, introduce her character well before the abduction. Maybe she has a special power or a secret that makes her a target. Then, create an interesting kidnapper. It could be a rival kingdom's agent or a mysterious figure with a hidden agenda. Build up the tension by showing the search for the daughter, with different characters having different motives in finding her.
There's also a chance that it's a random criminal organization. They might have heard about Caius's wealth and influence and decided that his daughter would be a valuable hostage. They could be planning to ransom her for a large sum of money or some other valuable resources.
I think Booster Gold could be a very unexpected hero. He's often seen as a bit of a joke, but during an Infinity Crisis, he could use his knowledge of future events (even if it's a bit hazy) to help the heroes. His time - traveling gadgets might also come in handy in dealing with the complex and chaotic situation of the crisis.
Well, one possible plot could be that the kidnappers are after some valuable secret that the Castle family has. Maybe it's a hidden treasure or a crucial piece of information related to a powerful organization. They hold the family hostage to force them to reveal it.
The kidnappers could be a group of hired mercenaries. Some powerful and wealthy person who has something to hide might have hired them. This person could be involved in illegal activities and Castle was on the verge of uncovering the truth, so they order the mercenaries to take Castle out of the picture.
The setting of the castle makes it interesting. Castles are often full of hidden passages, towers, and dungeons, which add an element of mystery and adventure to the kidnapping story.