The motives could be quite complex. Maybe the writer wants to show Serena's strength in a different way. By putting her in a seemingly helpless situation like being tied and gagged, it sets up a scenario where she has to use her wits and inner strength to get out of it. It could also be a form of character study, to see how Serena would react in a situation that is completely opposite to her normal, confident self in the Pokemon series. Additionally, it might be an attempt to add some edginess to the otherwise relatively family - friendly Pokemon fanfiction world.
I'm not really into this kind of rather extreme fanfiction idea about Serena in the Pokemon world. However, in general, fanfiction that involves such situations might be exploring themes of vulnerability or danger in a fictional and often not - so - serious way. But it's important to note that it should still adhere to ethical and respectful boundaries within the fan community.
A common motive could be power. The ones doing the brainwashing might want to use Serena's power as a trainer. For example, if she has a unique bond with her Pokemon that gives them extra strength, the villains would want to control that for their own gain.
Perhaps it's part of a prank that has gone too far. Some irresponsible people might think it's funny to play such a cruel 'joke' on her without realizing the seriousness of their actions. However, this is still a very wrong and illegal behavior.
One possible plotline could be a rescue scenario. Maybe her friends discover she's in this state and they have to outwit the captors. For example, Harry could use his invisibility cloak to get close and free her.
One reason could be that the reader has stumbled upon a secret. For example, if the story is set in a world of spies, the reader might have accidentally overheard some top - secret information and the villains want to keep him quiet.
One possible plot could be a mystery thriller. The girl is tied and gagged during a webcam session, and it's part of a ransom situation where the kidnappers are using the webcam to send messages to her family for money. Another could be in a horror context, where she is a victim of a deranged person who likes to terrify his victims on webcam.
Most likely not. The 'tied up' concept can easily cross into inappropriate or disturbing territory. In the Pokemon fanfiction world, there are much better and more positive stories about Serena that focus on her character development, her relationships with other characters like Ash and her Pokemon, and her adventures in the Pokemon universe.
One reason could be that the robbers are just looking for easy targets. They see the girl as an easy victim, perhaps because she is alone or looks vulnerable. So they tie and gag her to quickly take her belongings without any resistance.
Maybe she's a hostage in a robbery gone wrong, and the key is to the locked door where the robbers have stashed their loot. If she can free herself, she can alert the police.
Well, it could be a story about a prank gone wrong. Maybe Alice is playing a really extreme prank on Bella for some reason. For example, they could be in a competition within their group and this is Alice's way of getting an upper hand in a really silly way.