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.
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.
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 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 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.
One idea could be that Naruto's new fox features give him enhanced senses. He could use his improved hearing to detect hidden enemies or his sharpened sense of smell to track down missing ninjas. For example, in a mission to rescue a kidnapped villager, his new abilities help him find the secret lair of the enemy much faster than his teammates.
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.
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 might change. Sasuke might be more intrigued by Naruto's new form. He could see it as a sign of Naruto getting stronger in a different way. They might start to train together again to see if Sasuke can also develop some unique abilities. With Sakura, she might be more protective at first. She'd worry that Naruto's new features could make him a target. And Naruto would have to reassure her that he can take care of himself.
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.