Well, in this fanfic's plot, Naruto might be born into a more powerful and influential family that includes Tsunade. As the older brother, he has a natural sense of responsibility. He could be a prodigy in ninjutsu, and use his skills to protect Tsunade from the many threats in the Naruto world. Their relationship could also change the political landscape of the Hidden Leaf. For example, Naruto could be more involved in the decision - making processes of the village, and he could use his position to support Tsunade's ideas for reform. This would lead to a different kind of Hidden Leaf, where their combined influence shapes a new era.
One possible plot could be that Naruto grows up knowing about his relation to Tsunade. He could be trained in the art of medical ninjutsu by Tsunade's family, and then use his unique Uzumaki chakra to enhance it. As Tsunade's older brother, he might be very protective of her, especially after the death of her loved ones. He could play a key role in helping Tsunade become the Hokage, perhaps by convincing the village elders that she is the right choice, using his own influence as a respected ninja.
The plot could start with Naruto being separated from Tsunade at a young age. Maybe due to some sort of threat to their family. Naruto then grows up on his own, but discovers his relation to Tsunade later. When they reunite, there could be a lot of emotional turmoil. Naruto, as the older brother, could try to make up for the lost time. He could also use his knowledge of the future (if he has any, like in some time - travel fanfics) to prevent the tragedies that Tsunade has faced in the original story, such as the loss of Dan and Nawaki.
I'm not sure if there is a specific fanfic with that exact premise. However, in the Naruto canon, Naruto has no such relation to Tsunade. Fanfic writers might create such a story for the sake of exploring different character dynamics and plot twists. They could potentially rewrite the backstories to make it work, like having Naruto be from a different family lineage that is related to Tsunade's in some way.
One possible plot could be that Naruto, having grown up in a different world, suddenly discovers he has a sister in Erza's world. He travels to her world to protect her. Along the way, he has to learn about the magic in the Fairy Tail world while also teaching Erza some of his ninja skills. They could face a common enemy that threatens both their worlds, and their combined skills would be crucial in the fight.
One possible plot could be that Kyuubi protects Naruto from the very beginning. Instead of being seen as a monster, he's Naruto's brother and uses his power to shield Naruto from the villagers' hatred. Maybe they train together in secret, and Naruto becomes much stronger and more confident earlier in the story.
One possible plot could be that Naruto grows up with a different understanding of the Uchiha clan because of his connection to Shisui. He might be more involved in Uchiha affairs from an early age. For instance, he could try to prevent the Uchiha massacre, using his and Shisui's combined influence.
One possible plot could be that Anko uses her skills and knowledge to protect Naruto from the very beginning. Since she's his sister, she might be more involved in his training at a younger age. Maybe she teaches him some of her unique jutsu privately, which gives Naruto an edge over his peers. For example, she could teach him some of her snake - related jutsu, which Naruto could then modify to fit his own style.
One possible plot could be a training scenario. Naruto is blindfolded for a special training exercise, and Tsunade is overseeing it. Maybe Naruto has to rely on his other senses like hearing and smell to complete tasks while Tsunade observes and offers her tough - love guidance. It could also involve Naruto trying to prove himself to Tsunade in this unique situation.
Perhaps in the fanfic, Tsunade breaks Naruto's over - confidence. She might see him getting a bit too cocky and decides to put him in his place. She could do this through a series of tough training sessions or by making him face opponents that are stronger than he initially thought. This would be a way to humble Naruto and help him grow as a ninja.
One possible plot could be that Tsunade teaches Naruto advanced medical ninjutsu. Naruto, being a fast learner, quickly grasps the basic concepts but struggles with the more delicate aspects. Tsunade, with her tough - love teaching style, pushes Naruto to his limits until he finally masters it and is able to use medical ninjutsu in combat to heal his comrades.
One possibility is that there's a mission where Naruto and Tsunade are paired together. Along the way, Naruto gets into some mischief and tries to tickle Tsunade to cheer her up when she's feeling stressed about the mission. Tsunade initially acts annoyed but then realizes it's all in good fun. This could also be a way for Naruto to bond more with Tsunade on a personal level, rather than just as the Hokage and a ninja.
Well, it could be about Tsunade and Naruto sharing a moment over a glass of lemonaid. Maybe Naruto is trying to cheer Tsunade up after a tough day in the village. They start talking about their dreams and Naruto's journey to become Hokage, and the lemonaid is just a simple, refreshing backdrop to their conversation.