Obsession is a very common theme. Yandere characters in these fanfictions are often overly obsessed with a particular Harry Potter character, which drives their actions.
One popular 'Harry Potter x Yandere fanfiction' is 'The Yandere Potter'. It has an interesting take on how a yandere - like character could exist in the Harry Potter universe. The story plays with the idea of extreme obsession and how it could disrupt the normal Hogwarts life.
One possible plot could be Luna being overly possessive of Harry in a way that surprises him. For example, she might try to keep him away from his other friends at Hogwarts, thinking they are a bad influence on him. Another plot could be Luna using her unique magical abilities in rather extreme ways to protect Harry from perceived threats.
Harry is the most important character interacting with Yandere Cho. But we can't forget about Luna Lovegood. Luna might notice Cho's abnormal behavior and try to warn Harry. Also, Cedric Diggory's memory could play a role. Since Cho was Cedric's girlfriend before, his memory might haunt her and fuel her yandere tendencies, and this could be noticed by characters like Harry when they interact with her.
She becomes more possessive. Normally, Luna is a dreamy and somewhat detached character, but in yandere fanfictions, she'll be much more clingy and protective of Harry.
Harry Potter is obviously a central character. Then there could be female characters like Hermione who might take on yandere traits in the fanfiction. Also, characters like Ginny Weasley could be part of the harem with yandere - like behaviors towards Harry.
One interesting story could be 'The Yandere's Obsession in Hogwarts'. It might revolve around a character who has an extreme, possessive love for another within the magical world of Harry Potter. Maybe a Slytherin student being overly possessive of a Gryffindor they're in love with.
One possible plotline could be Cho becoming overly possessive of Harry due to her yandere nature. For example, she might try to isolate him from his friends, believing that they are taking too much of his time away from her. Another plotline could involve Cho using her knowledge of the wizarding world in unexpected ways to keep Harry close to her, like using spells that she shouldn't be using in a non - life - threatening situation just to ensure Harry stays by her side.
In some plots, the yandere elements add a lot of drama to the story. For instance, there could be a love triangle within the harem where two yandere characters are vying for Harry's attention. This could lead to all sorts of magical mayhem. One might try to use a love potion on Harry while the other tries to counter it with their own magic, all while Harry is in the middle, completely oblivious to the full extent of their 'yandere' natures.
Yandere characters in these fanfictions are likely to go to great lengths to be close to their love interest. They may manipulate situations or use magic to ensure they are always around the person they're obsessed with. They might also be very secretive about their true intentions and feelings, acting all nice on the surface but having a darker side.