In a more supernatural take, a vengeful spirit could be the kidnapper. The castle has a dark history, and Alexis unknowingly disturbs the spirit. So, the spirit kidnaps her and takes her to a dimension within the castle that exists between the living world and the afterlife. Her friends would then have to find a way to communicate with the spirit and break the curse to rescue her.
There might be. Fanfic writers often come up with all sorts of creative and sometimes dark scenarios for their favorite characters. Since Alexis Castle is a character from the 'Castle' series, it's possible that in the large pool of fanfic, there is one that involves her being kidnapped. However, without specifically looking through relevant fanfic websites, it's hard to say for certain.
One idea could be that Alexis is a princess in the castle. A rival kingdom's forces kidnap her to use as a bargaining chip in a territorial dispute. Her loyal servants then set out on a secret mission to rescue her, facing various challenges along the way.
Fanfiction.net is also a place to check. Do a search for 'Alexis Castle' and then carefully read through the descriptions of the stories that come up. Another option is to check any dedicated 'Castle' - related fan forums or communities. Sometimes fans share their own fanfics or know where to find specific types of stories, like one about Alexis being kidnapped.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
Maybe it's a group of villains working together. They could be from different factions but united by the idea that Izuku has the potential to become a huge threat to their plans. They might kidnap him to either eliminate him or turn him to their side. Also, it could be a lone, obsessed fan of a villain who wants to prove themselves by bringing Izuku to their idol.