Perhaps as a sign of rebellion. If Naruto has some issues with the established norms in his village or with the ninja system in general in the fanfiction, getting piercings could be his way of defying authority and showing that he won't be restricted by the traditional ways.
Yes, it can. Piercings can add an edgy or unique look to Naruto in fanfiction. It might make him seem more rebellious or cool, different from his original appearance in the official series.
One possible storyline could be that Naruto gets tattoos and piercings as a sign of his growth and independence. He might use them to show his newfound confidence and to distinguish himself from his previous self. For example, he could get a tattoo of the Nine - Tails on his arm to symbolize his connection with the powerful beast inside him.
In many fanfictions, Naruto might use new jutsus or a deeper understanding of his own power. Maybe he taps into a more powerful form of the Nine - Tails' chakra and combines it with his Sage Mode in a unique way to overpower Pain.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
His fighting style could become more aggressive. Without the fear of pain, he might be willing to use more extreme techniques that would otherwise be too dangerous because of the pain they cause to the user. He could push his body to the limit more often.
Konan could be really interesting. In some fanfictions, she might have a different backstory or motivation that affects the Naruto vs Pain fight. For example, she could be secretly trying to stop Pain from going too far in his destruction of the Leaf Village.
Jiraiya could be a great addition. Since he was Pain's teacher, his presence could add a lot of depth to the story. He could offer insights into Pain's past and also help Naruto understand Pain better.
Sakura might be initially shocked. She has always known Naruto as the go - lucky ninja, and the tattoos and piercings would be a big change. She might scold him at first but then come to understand his reasons behind it.