Well, a really interesting plot twist could be that the older sister has a secret past she's been hiding from Levi all along. Maybe she was involved with an enemy group in her youth, and now they're coming back to use her against Levi. This would create a huge rift between them at first, as Levi would feel betrayed. But then, as he learns more about her reasons and her true loyalty, they could come back together stronger than ever. And on a different note, it could be that a magical curse has been placed on their relationship, and they have to overcome it in order to be together.
One way to develop their relationship uniquely is to focus on their shared traumas. Maybe they both witnessed something terrible in their past that only they understand. This common ground can be used to create a deeper bond as they heal together. Another way is to give the older sister a special skill or knowledge that Levi respects and needs. For example, she could be an expert in a type of combat that Levi is unfamiliar with, and he has to rely on her for training.
One idea could be that the reader is a new recruit in Levi's squad. Levi's wife, being kind - hearted, takes the reader under her wing. She helps the reader with training and they form a deep friendship. As time passes, the reader discovers some hidden secrets about Levi's past through his wife, which adds an element of mystery to the story.
Maybe Levi is a criminal in the sense that he's part of an underground rebellion against an unjust regime. Eren, who is initially on the side of the establishment, meets Levi and gradually comes to understand his motives. Their relationship could develop as Eren has to decide whether to side with Levi or stay loyal to the regime, all while a romantic connection forms between them.
One possible plot could be that Draco's older sister, let's call her Astoria for example, starts to develop feelings for Harry while trying to bridge the gap between the Slytherin and Gryffindor houses. She might see the good in Harry that Draco refuses to acknowledge at first. This could lead to a series of events where she has to choose between her family's traditional views and her new - found respect and love for Harry.
Well, in a Rogue Titan x Levi fanfic, the plot could start with the Rogue Titan being a mystery. Levi is assigned to figure out what this Rogue Titan is all about. As he gets closer to it, he realizes that the Titan has a level of intelligence that he didn't expect. The Rogue Titan, in turn, seems to recognize Levi in some way. Their relationship could be a slow - burn one, where they gradually learn to trust each other. Along the way, they might face challenges from the military leadership who don't trust the Rogue Titan. And there could be a big reveal at the end about the true nature of the Rogue Titan and how it ties into the overall story of Attack on Titan, like maybe it's a key to unlocking a new power or a secret about the Titans' origin.
The theme of self - discovery is also present. Through the relationship with Levi, the reader often discovers new things about themselves. Levi's strict and disciplined nature can inspire the reader to grow and become stronger, both physically and emotionally. This is shown in many fanfics where the reader overcomes their own insecurities with Levi's help.
Well, one possible plot twist could be that Bob, who was thought to be an ordinary reader, actually has some sort of superpower. Maybe he can read the thoughts of the characters in the books he reads. This would completely change the dynamic of the fanfic as he could start interacting with the fictional world in unexpected ways.
The plot could be simple. A storm hits while they are at the Survey Corps headquarters. Levi is in his room trying to ignore the thunder, but it's getting to him. Erwin, who also has a fear of thunder, comes to check on him. They end up sitting together, sharing stories about how they first realized they were scared of thunder, and this brings them closer.
The unique relationship. Levi is a complex character, and when paired with a titan - reader, it creates a contrast. Titans are usually seen as enemies, so seeing how they interact in a fanfic, whether it's a friendship or more, is really engaging.
Well, in many of these fanfics, there's a theme of the badass reader breaking Levi's routine. Levi is used to a certain way of life and the reader comes in and disrupts it in a good way. It leads to personal growth for both characters. There's also the theme of loyalty. As they face challenges together, their loyalty to each other deepens, whether it's in battle or in dealing with internal politics within their group.