The Bnha universe's reaction would be mixed. Those who are into complex stories, like Todoroki, might study the relationships in Hamilton fanfiction. But others, like Mineta, might not be interested at all because it doesn't have much to do with his usual interests. Overall, it would depend on the individual characters' personalities and interests.
Some would be inspired. Characters like All Might might see Peter Parker's journey as similar to his own in some ways. Both are trying to do good in a world full of challenges. All Might could appreciate Peter's determination to protect his city despite having a normal life outside of being a hero.
For heroic - centric fanfictions, they would likely be disdainful. Since they oppose heroes, seeing stories that glorify heroes would annoy them. They might scoff at the idealized versions of heroes in those fanfictions.
I'm not really sure what you mean by 'kanao comics hentai'. It might refer to some adult-oriented or inappropriate content related to a particular character.
In the manga, Tanjiro and Kanao don't end up getting married. The story focuses more on their battles and character development rather than romantic pairings.
One great tanjiro and kanao fanfiction could be where they go on a journey together to find a rare herb for an ailing comrade. In this story, their relationship deepens as they face various obstacles in the forest.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.