One origin could be a curse placed on Naruto. In the Naruto universe, there are many powerful curses. This curse could be related to the nine - tailed fox's history or some ancient feud. It might be that a long - forgotten clan or entity has put this curse on Naruto as a form of revenge or to serve their own dark purposes. Or it could be a natural evolution of Naruto's connection to the nine - tailed fox as he reaches a new level of power, but his body isn't fully able to handle it yet, so it manifests in these physical changes.
One possible reason could be a strange side - effect of his connection with the Nine - Tails. Since he has the Nine - Tails' chakra within him, in the fanfiction world, this chakra might start to physically manifest in the form of fox ears and a tail during his time in Wave. Maybe the unique environment or a specific event in Wave triggered this abnormal transformation.
Naruto's daily activities would be affected. For instance, he would need to hide his new features, which could be difficult. Hunting and stealth missions would become more challenging as his new appendages might give him away. Also, his relationship with his Sensei might change. His Sensei would need to figure out how to train him while dealing with these new physical aspects. His combat style would also have to adapt. He could use his new fox - like abilities, such as enhanced hearing with the ears, but he would also need to learn how to control his balance better with the tail.
One possible plot could be that Naruto discovers a new jutsu or is under a curse that causes him to grow fox ears and a tail. This new form might make his friends initially confused or even a bit scared. But as the story progresses, he learns to control this new part of himself and uses it to his advantage in battles, like having enhanced senses from the ears and better balance with the tail.
In a fanfiction, Female Naruto with fox ears and tail might have a special connection to the Nine - Tailed Fox spirit. The fox ears and tail could be a sign of a deeper bond that she has with the tailed beast. This could lead to new jutsu or abilities that are related to both her femininity and the fox - like nature. For example, she could create illusions that are fox - themed or use her tail as a weapon in a more graceful way than normal attacks.
A possible plot is that baby Naruto with his fox ears and tail is found by a group of friendly animals in the forest. They take him in and protect him from the villagers who might be scared of his appearance. As he grows up among the animals, he learns their ways of survival and also starts to discover his own ninja - like abilities that are related to his fox features.
In a more comical plot, Naruto's fox ears grow randomly one day. He has to go about his normal life in Konoha with these ears, causing all sorts of chaos. People keep mistaking him for a new kind of summon or some strange experiment gone wrong. His friends have to help him keep his spirits up while they search for a way to reverse it, if that's what he wants.
The fanfiction could be set in an alternate universe where having fox ears and tail is normal for some ninjas. Female Naruto would be part of a group that is trained specifically to use their fox - like features in combat. There could be rival groups with different animal - like traits, and she has to fight for the honor of her fox - clan.
His relationship with Sasuke could be affected. Sasuke might be initially intrigued by Naruto's new look but also see it as a sign of Naruto getting stronger without him. This could fuel his rivalry even more.
Well, perhaps it could be part of a transformation arc in the fanfiction. Izuku starts growing the fox tail gradually, and it's related to an ancient power or a prophecy. As the tail grows, he attracts the attention of certain characters who either want to help him or use him for their own gain. He might have to go on a journey to find out the origin of this new feature.
One idea could be that Naruto discovers his fox ears and tail have special powers. For example, the ears can detect danger from afar, like an enhanced sense of hearing. The tail could be used for balance during his high - speed ninja moves, making him even more formidable in combat.