One possible reason could be that Naruto is hiding his face to conceal his emotions. In SasuNaru fanfiction, their relationship is often complex and full of unspoken feelings. Naruto might be afraid that Sasuke will read his true feelings if he shows his face. For example, if Naruto has developed deeper romantic feelings for Sasuke, he may not be ready to expose them, so he hides his face as a form of self - protection.
Maybe Naruto hides his face because he's ashamed or guilty in the context of SasuNaru stories. Suppose Naruto made a mistake that he thinks will make Sasuke disappointed in him. Hiding his face would be a way to avoid seeing Sasuke's reaction directly. It could also be related to some power - related incident. For instance, if Naruto has tapped into a new, dangerous power that he can't fully control yet, he may hide his face to keep Sasuke from getting involved or worried.
One possible reason could be that he is on a secret mission and doesn't want to be recognized. He might be infiltrating an enemy organization or spying on someone. Another reason could be that he has some new power or mark on his face that he wants to keep hidden until he fully understands or masters it.
On the other hand, Sasuke could be worried. He may think that Naruto is in some kind of trouble or danger. Given their history and the care they have for each other in the SasuNaru dynamic, Sasuke's concern could lead him to be more protective. He might start investigating the situation on his own to figure out why Naruto is hiding his face and ensure his safety.
Perhaps he's trying to protect himself from some kind of danger. There could be an enemy who is looking for him and by covering his face, he can avoid being detected easily. It could also be a part of a new jutsu or power he has discovered where covering his face is necessary for its activation or control.
He might want to protect himself. If others know how smart he is, he could become a target. Enemies might see him as a greater threat and go after him more aggressively.
Perhaps he has hidden the full extent of his Sage Mode abilities. He might be able to stay in Sage Mode for a much longer time than he has shown so far, or have some other undisclosed powers within that mode, like the ability to sense enemies from much greater distances.
Perhaps Naruto has a big dream, like becoming Hokage. She might think that in a world where male leadership is more common, hiding her gender would increase her chances of being taken seriously. People might be more likely to believe in a male's ability to lead rather than a girl's, so she hides it to pursue her goal more effectively.
Perhaps Naruto is waiting for the right moment to reveal his skills for maximum impact. He could be planning a big surprise for his enemies or wants to make a grand entrance at a crucial time. For example, in a major battle against a powerful foe, he could suddenly unleash all his hidden skills and turn the tide of the battle, which would be more effective than showing them off gradually.
Another possibility is that Naruto has a plan that requires him to keep his skills hidden from everyone, including Hinata, for a while. He could be waiting for the right moment to reveal them and use them to surprise and save the day, especially when it comes to protecting Hinata from a big threat that he anticipates.
Sasuke could be one of the people who help him. Despite their past differences, they have a deep bond. Sasuke has his own skills and knowledge that could be useful in Naruto's situation, like his Sharingan abilities which could help in detecting enemies while Naruto is hiding his face.
Maybe Naruto hides his genius to avoid unwanted attention. He might be afraid of being targeted by enemies or those who would use his abilities for bad. For example, in the original series, he already had a lot of people after him because of the Nine - Tails. By hiding his genius, he could be trying to keep a low profile and train in secret to become even stronger without interference.