Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.
Sakura would be devastated. She has deep feelings for both Naruto and Sasuke. She might turn against Sasuke completely and try to take revenge herself. Kakashi would be in shock. He has mentored both and would likely feel like he failed in some way. As for the other ninja from the Hidden Leaf, they would be angry and mournful at the loss of their hero Naruto.
Well, it depends on the nature of the fanfiction. Some anime characters might be amused if the fanfiction is light - hearted and full of comical situations. For example, if it's a fanfic about a cooking competition among the characters from a shonen anime, they might react with good - natured laughter and competitiveness. They could also be confused if the fanfiction takes their story in a very strange or unexpected direction, like suddenly making a character from a historical anime end up in a space adventure.
Naruto himself would likely be outraged. He has his own goals and reasons for his relationship with Sasuke, and being labeled as a 'fan' by Ino would be offensive to his pride. He'd probably try to set the record straight right away.
Sakura would be completely devastated. She's been in love with Sasuke for so long and has a deep bond with Naruto too. She might go into a deep depression and lose her will to be a ninja for a while.
Naruko herself would probably be shocked at first, but then she would start trying to reason with Sasuke. She would ask him why he's doing this and try to make him see that whatever he's thinking is wrong. She's a strong - willed character and wouldn't just sit there passively.
Sasuke would probably be very guarded at first. He's not used to dealing with something so innocent and vulnerable. He might be suspicious of the baby's presence and think it's some sort of trap.
Sasuke would be furious. He's always been protective of Naruto in his own way. He would immediately start looking for her, using all his skills and resources. He might even go on a rampage if he thought Naruto was in serious danger.
On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.
Sasuke might be initially surprised. He is used to the Sakura from his past, so seeing a different, more mature future Sakura could catch him off guard.