In Avengers fanfiction, Thor's sister can be depicted in many ways. One common way is as a mysterious figure. She might have secrets about Asgard that even Thor doesn't know. Writers also tend to give her unique relationships with other Avengers members. For instance, she could have a special bond with Loki, which adds an interesting dynamic to the story.
In fanfiction, Thor's sister can be a really interesting character. She might have powers similar to Thor's, like control over lightning or super strength. Writers often create her with a unique personality, perhaps she's more cunning or has a different moral compass than Thor. This adds new dimensions to the Avengers' stories in fanfiction.
In fanfiction, her character is distinct from Thor and Loki. Thor is associated with honor and strength in combat, Loki with his slyness and intelligence in manipulation. Their sister can be seen as a bridge between their two extremes. She may not be as physically strong as Thor but has her own form of power, perhaps in the realm of diplomacy or magic that is different from Loki's. For example, she could use her magic to create harmony rather than chaos like Loki. Also, her motivation might be different. Instead of Thor's quest for glory or Loki's desire for power, she could be driven by a need to protect her family and the realms in a more holistic way.
Well, one possible plot could be that Thor and Loki's little sister has some special powers that are a mix of their own. She might be caught in the middle of their feuds, trying to bring peace between them. For example, she could use her powers to stop Loki from his mischievous plans that would anger Thor, while also making Thor understand Loki's motives better.
It could be Malekith. He might see the sister as a way to get to Thor and Loki and gain power over Asgard. Since she's a new and potentially vulnerable target, he could try to use her in his dark schemes.
There are quite a few fanfictions out there. In some, she is a powerful sorceress like Loki. For example, in 'Sorceress Sister', she uses her magic to help Thor in battles against dark forces. Some fanfictions also explore her relationship with the other Asgardians. In 'Asgardian Bonds', she forms new alliances and friendships within the realm while also reconnecting with her brothers.