One possible plotline could be that Naruto leaves with Tsunade on a mission to find a rare herb that is said to have the power to heal any wound. Along the way, they face various enemies who also want the herb for their own malicious purposes. Naruto's skills and Tsunade's wisdom would be put to the test as they battle their way through.
Another plot could be that Tsunade decides to take Naruto on a journey to meet some of the most powerful ninjas in other villages for training. Naruto is excited to learn new techniques. On their travels, they encounter different cultures and ninjutsu styles. Tsunade acts as a guide and mentor, and Naruto has to adapt quickly to the new challenges presented by these foreign ninjas.
Another plotline might be that Tsunade is affected by some kind of dark magic or genjutsu that makes her turn against Naruto. Naruto, realizing it's not really Tsunade's fault, still has to fight her to try and break the spell. He could call upon his friends for help, but in the end, it's his determination and his bond with Tsunade that helps him reach her and break the control.
One possible plotline could be that Naruto discovers some long - lost secret about his mother Kushina and decides to leave the village with her to explore this mystery further. Maybe they are on a quest to find some hidden power or knowledge that is related to Kushina's Uzumaki heritage.
One possible plotline could be that Naruto is somehow tricked into leaving with Kisami and Itachi. Maybe they convince him that there is a greater threat that only they can help him face. Along the way, Naruto might start to learn more about their motives and the secrets of their pasts. He could also be in a position where he has to use his own skills and values to influence them.
One possible plotline could be Tsunade teaching Naruto more about medical ninjutsu. Since she is a great medical ninja, she could pass on her knowledge to Naruto, which would add a new dimension to his skills. Another aspect could be her guiding Naruto in leadership, as she is the Hokage. This might help Naruto in his future role as a leader in the village.
Perhaps in the fanfic, Futa is a new character in the Naruto world who has some sort of connection to Tsunade's past. Naruto gets caught in the middle of their relationship. It could involve Futa having some secrets that Tsunade is desperate to know, and Naruto helps in uncovering those secrets while also growing as a ninja himself.
One possible plotline could be Naruto feeling extremely lost and having to find his own way to become stronger without their guidance. He might start to question his own worth and abilities, but then he could meet new allies who help him realize his true potential on his own. For example, he could encounter some other outcast ninja who have their own unique skills and together they form a new kind of ninja team that challenges the traditional ways of the ninja world.
One possible plotline could be Naruto growing up with a different set of skills influenced by Tsunade. Since Tsunade is known for her medical ninjutsu and super strength, Naruto might have an earlier inclination towards medical skills. He could be trained in a more balanced way between combat and healing. For example, in battles, he not only uses his regular jutsu but also heals his comrades instantly on the spot. Another aspect could be the political influence. Tsunade being an important figure in the village, Naruto might have a different standing in the village hierarchy from the start and face different kinds of challenges related to leadership and decision - making.
One possible plotline could be that Naruto has his own mission or purpose that is in conflict with Rias's plans or the world she is a part of. So, he decides to leave to pursue his own goals. For example, he might need to go back to the Hidden Leaf to protect it from a new threat that only he can handle. Another could be that Naruto discovers some dark secret about Rias or her group and can't stay with them any longer. He values his own moral code too much to be associated with something he deems wrong.
One possible plotline could be that Naruto discovers a new form of chakra control that gives him an edge over Tsunade. He uses this to take charge in a dangerous mission where Tsunade was initially leading.
Well, if there is such a fanfiction, perhaps there's a plot where some sort of magic or jutsu affects their minds. Naruto might be under a spell that makes him see Tsunade in a romantic way, and Tsunade, being affected by a similar or related jutsu, reciprocates. And so, in this strange situation, they end up in a situation where they'sleep together'. But this is all just speculation based on what could be in a very unorthodox fanfiction.