One possible scenario in this fanfiction could be that Naruto's sister constantly interferes in his ninja training. She might think the training is too dangerous for him and tries to stop him from taking on challenging missions. For example, when Naruto is about to go on a mission to the Land of Waves, his sister could try to convince the Hokage not to send him, believing that he's not ready yet. This would add a whole new layer of conflict to the story, not just between Naruto and his sister, but also with the village's expectations of Naruto as a ninja.
One common theme is loyalty. The overprotective sister is often extremely loyal to her sibling, willing to go to great lengths to keep them safe. For example, she might put herself in danger without hesitation.
There are quite a few out there. In a lot of these 'Naruto overprotective sister fanfiction' stories, the sister is usually a new character created by the author. Her overprotective behavior can stem from Naruto's status as a jinchuriki or simply because she loves him dearly. For instance, she might train secretly to be strong enough to protect him. These fanfictions can offer a different perspective on Naruto's growth and the relationships he forms within the Naruto universe.
It's appealing because it adds a new family - related angle. Naruto didn't really have a strong family presence in the original, so having an overprotective sister fills that gap.
The plot could start with Naruto's sister being introduced suddenly. Maybe she was away for a long time, and now she's back and realizes how much Naruto has grown up but also how much danger he's in as a ninja. She then starts to follow him around everywhere, much to Naruto's annoyance. For instance, she might show up during his battles and try to jump in to protect him even when he doesn't need it.
There's also 'Overprotective Ruby' which focuses on Ruby being overly protective of her sister or friends. It has some really interesting scenarios where Ruby's protectiveness gets her into both comical and tense situations. This fanfiction does a great job of exploring Ruby's character in this context.
One interesting fanfic could be 'Naruto's Protective Sister: Konoha's Guardian'. In this fic, the sister uses her unique jutsu skills to protect Naruto from various threats. Another might be 'Sisterly Love for Naruto' where she constantly fends off those who might harm Naruto due to his jinchuriki status.
A common theme in 'overprotective big sister x reader fanfiction' is conflict. There can be external conflicts like other characters trying to harm the reader, which the sister has to fend off. But there are also internal conflicts, such as the sister's struggle between being overprotective and allowing the reader to have their own experiences. This creates a lot of drama and tension in the story. Additionally, the theme of unconditional love is prevalent. The sister loves the reader without any conditions, and this love drives her actions throughout the story, whether it's scolding the reader for being reckless or celebrating their achievements.
Often, in fanfiction, you'll see Naruto being overprotective when his sister is about to go on a mission. He'll try to convince her not to go, or if she does, he'll insist on going with her. He might also be overly cautious when she's interacting with new people or in new environments. For example, at a village festival, he'll keep a close eye on her to make sure no one takes advantage of her. And if she gets injured, even slightly, Naruto will go into full - on protective mode and might even overreact a bit.
Finding good 'overprotective big sister x reader fanfiction' can take a bit of time. You could also ask in relevant fan communities on social media platforms. For example, on Tumblr, there are often groups dedicated to different types of fanfiction. They might be able to point you to some hidden gems. The best part about this type of fanfiction is that it often explores the relationship dynamics between the two characters in a very emotional and sometimes comical way.