Well, she could be a prodigy in medical ninjutsu. In the fanfiction, she decides to use her skills to help both Naruto and Sasuke. She secretly treats Naruto's wounds after his tough battles, and at the same time, she tries to convince Sasuke to come back to the village. Her actions cause a stir in the ninja world as everyone wonders who this mysterious sister of Sasuke is and what her motives are.
One interesting plot could be that Sasuke's sister has a unique power that both Naruto and Sasuke are unaware of. She could be a key to uniting the Hidden Leaf Village and the Uchiha clan in a new way. Maybe she has the power to heal the rift between the village and the remaining Uchihas after the massacre. And as Naruto and Sasuke are on their own paths of growth, she emerges as a bridge between them, helping them understand each other better.
A possible plot is that Sasuke's sister is kidnapped by an unknown enemy. Naruto and Sasuke then have to put aside their differences and work together to rescue her. During this adventure, they discover secrets about her past and her hidden abilities. For example, she might have a special connection to an ancient and powerful jutsu. This journey not only saves her but also strengthens the bond between Naruto and Sasuke.
A plot could be that Sasuke's sister discovers a hidden power within herself that could either save or destroy the ninja world. Naruto and Sasuke have different views on how to handle this situation. Naruto wants to help her control it for good, while Sasuke is more cautious. This creates a conflict between them and also gives the sister a central role in the story, as she has to decide her own fate.
Perhaps in the fanfiction, the bathhouse is a magical place where Naruto and Sasuke's memories are shared. They see each other's childhood traumas and it makes them understand each other on a deeper level. They then make a pact to protect each other more fiercely in the future while in the relaxing atmosphere of the bathhouse.
Perhaps the fanfiction could focus on the little sister's own journey of self - discovery. Growing up in the shadow of her two powerful brothers, she might strive to find her own identity and power. She could train secretly and then surprise everyone with her newfound abilities. And in the process, Sasuke and Itachi might have to deal with their own feelings about their sister growing up and becoming independent.
A great plot could be that Sasuke, in the reversed role, is the village's hope. He is the one everyone looks up to. Naruto, with a dark past that led him to be bitter, is on a path of self - destruction. Sasuke tries to stop Naruto from causing harm to himself and others. He uses his own positive energy and the power of friendship to make Naruto realize that there is more to life than revenge. This would be a complete role reversal from the original Naruto and Sasuke dynamic.
A cool plot could be that a new villain emerges who has the ability to nullify the powers of the Sharingan and Rinnegan. Naruto and Sasuke have to find a way to overcome this without relying solely on their eye powers. They might discover ancient techniques that their predecessors used before the existence of these eye powers.
A plot development could be Naruto in Sasuke's body being forced to participate in a mission that was originally meant for Sasuke. Since Naruto has his own set of values and beliefs, he might approach the mission very differently. He could try to find non - violent solutions where Sasuke might have been more inclined to use force. This would not only cause confusion among his teammates but also make his enemies underestimate or misjudge him. And as Naruto tries to adapt to Sasuke's body, he might accidentally unlock some hidden powers or abilities that Sasuke himself was not aware of.
She could have a special connection to an ancient ninja art. Naruto wants to learn from her to become stronger and Sasuke is intrigued as well. This causes some competition between them at first. But then they realize that they can all benefit from her knowledge. Also, her existence could stir up old family secrets. Maybe she has a connection to the Uchiha clan that Sasuke doesn't know about, and this revelation could change his perception of his family and his relationship with Naruto.
Well, the plot could involve Sasuke finding Naruto in a weakened state and taking him in as a pet. Naruto might initially resist but then gradually get used to the situation. There could be parts where Sasuke trains Naruto in a different way compared to the normal ninja training. And maybe there are other characters' reactions to this strange relationship, like Sakura being completely bewildered.
Well, an interesting plot point might be how their different ninja abilities are influenced by their brotherly bond. Since Naruto has the Nine - Tails, he could train Sasuke to better handle his Sharingan. And in terms of the story, when Sasuke is faced with the truth about Itachi, Naruto as his older brother could be there to comfort him and help him make a more rational decision instead of just seeking revenge. There could also be a plot point where the village starts to suspect their relation and how they deal with that suspicion together.
One possible plot could be that Naruto's twin sister has a unique and powerful ninja ability that she has to keep hidden from the village at first. Maybe she can communicate with animals in a very special way, like having full conversations and understanding their emotions. As the story progresses, she uses this ability to help Naruto and their friends in various missions, for example, finding a lost ninja by asking the birds for directions.