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.
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.
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.
One interesting 'Harry Potter x Luna' fanfiction could be 'Luna's Hidden Adventures'. In it, Luna and Harry go on a secret quest in the Forbidden Forest. They discover a new magical creature that has been hidden there for centuries. Another is 'The Bond between Harry and Luna' which explores their relationship in a more emotional and in - depth way, showing how they support each other through tough times at Hogwarts.
Maybe the story could revolve around how Harry and Luna protect their baby from dark forces that still lurk in the wizarding world. They would use their unique skills and knowledge to keep the baby safe. For example, Luna's ability to see things others can't might help them detect hidden threats. Harry, with his combat skills, would be ready to fight off any attackers. Their friends like Ron and Hermione could also play important roles in this story, perhaps helping to babysit or providing magical protection for the baby.
A possible plot is that Makoto gradually realizes Haru's yandere nature. At first, he might be confused and a bit scared. But as he truly loves Haru, he tries to understand and help Haru overcome these tendencies. This could lead to a journey of self - discovery for both of them as they navigate their relationship.
One interesting plot could be Hisoka becoming overly possessive when the reader starts getting close to another character. He might start to eliminate any potential threats in a very Hisoka - like, unpredictable way.
A plot might involve a yandere legendary Pokemon like Mewtwo falling for the reader. Mewtwo could use its psychic powers to manipulate situations so that the reader has no choice but to be with it. It might create illusions to scare away potential rivals or even control the reader's thoughts to some extent.
One plot could be that in a dark version, Harry and Luna team up to expose a hidden, evil regime within the wizarding world that has been secretly controlling things from the shadows. They might have to use some rather unorthodox and darker magic to do so.
There might be a fanfiction where Harry Potter time - travels and meets a younger Lily Luna Potter. He then has to protect her from a new threat while also learning about how different she is from what he expected. In this story, their relationship could develop in a really interesting way, like Harry becoming a mentor to her in a way he couldn't be to his younger self.
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.