Maybe it's because Nico has some complex and often dark or angsty aspects to his character in the source material. Fanfiction might explore these in extreme ways that can be disturbing to some readers.
I think it's because Nico is a character with a lot of depth. Fanfiction might delve too deep into his psyche and expose some very raw and disturbing aspects that people aren't prepared for. It could also be that the way some fanfiction writers portray his relationships, especially the more controversial ones, makes people scared to engage with it.
Yes, there are likely to be such fanfictions. In the Naruto series, Sakura can be quite assertive and strong - willed. Her temper and skills could potentially be a source of intimidation for Naruto in the creative interpretations of fanfiction writers.
In many Naruto fanfictions, Naruto might have developed some new and unique abilities that the Kyuubi has no defense against. This could make the Kyuubi, which is used to being a dominant and feared force, feel vulnerable. For example, if Naruto has the power to nullify the Kyuubi's chakra or to manipulate it in a way that goes against the Kyuubi's will, it would be a cause for fear.
It could also be related to his jinchuriki status. The Nine - Tails' chakra within him might make him feel that being touched could lead to the unleashing of that power accidentally, and he's afraid of harming others or losing control. So, to avoid any such situations, he's scared of being touched.
I'm sorry, I can't directly recommend a specific 'Naruto Scared of Sakura' fanfiction. But you can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Use the keywords 'Naruto scared of Sakura' in the search bar, and you should be able to find relevant stories if they exist.
It could be that his new 'godlike' power has isolated him. People might be either in awe or afraid of him, and he may feel that he can't be close to others as before. So he acts coldly as a defense mechanism.
Maybe it's the high expectations that Naruto has for Boruto. Naruto is a hero, and he might unknowingly put a lot of pressure on Boruto to follow in his footsteps. Boruto could be scared of not being able to meet those expectations.
They could start by having an honest conversation. Naruto could sit down with Boruto and just listen to his fears without judgment. This would be a first step in building a better relationship.
In fanfiction, Naruto is often loved by everyone because of his unwavering determination. He never gives up, no matter how tough the situation is. This trait is inspiring and makes other characters in the fanfiction world respect and love him.