Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
Another possibility is that Naruto and Yang have a special bond. This bond could be formed through shared experiences, like they both went through a difficult training together or survived a life - threatening situation. Because of this special connection, Naruto feels indebted to Yang or simply values Yang's life more than his own arm.
In this fanfiction, perhaps Naruto confronts Kushina about the secrets she kept from him. For example, if there were things she knew about his powers or his destiny that she didn't share before her death. He could be feeling betrayed and wants answers.
It could be that there is no traditional antagonist. The conflict might be internal, with Naruto's own emotions towards Kushina being the main source of drama.
Perhaps he meets an ancient sage or a hermit who has the knowledge of regeneration. This character could guide Shino and provide him with the necessary herbs, incantations or mental exercises to regrow his arm. It could be a spiritual journey for Shino as he has to prove himself worthy to receive this knowledge.
In some fanfictions, Shino might regenerate his arm through special ninja techniques. For example, he could use a rare form of medical ninjutsu that he discovers or is taught. Maybe there's a hidden scroll in his clan's archives that reveals a way to regrow limbs.
With his friends, it might strengthen their bonds. They would be more protective of him and help him in his daily tasks. For example, Sasuke might become more empathetic towards Naruto's situation. His relationship with Hinata could also change. Hinata would be very concerned and would constantly be by his side, offering emotional support.
Naruto would need to train harder to make up for the lack of an arm. He could develop new jutsus or modify existing ones. Maybe he would start using more wind - based attacks that can be controlled with just one arm and his chakra control. Also, his friends might help him by creating prosthetics or assisting him in battles.
His combat abilities are severely affected. Naruto is known for his physical combat skills combined with his chakra - based attacks. Without his right arm, his punches and grappling moves are limited. He can't hold his kunai or shuriken as effectively in combat, which reduces his ranged attack options.
One possible storyline could be Naruto's struggle to adapt to his new physical state. He might have to relearn all his ninja techniques, now that he's missing an arm. For example, he'd have to find new ways to perform his signature Rasengan. Another could be his journey of self - acceptance, dealing with feelings of inadequacy compared to his former self. He could also face challenges from enemies who see his injury as a weakness and try to take advantage of it.