They might be really surprised. Some characters could be flattered by the positive portrayals in the fanfiction, while others might be a bit annoyed if their personalities are misrepresented. For example, Deku might be shy but also happy to see new interpretations of his growth journey.
Shigaraki would likely be disgusted if the fanfic made him seem weak or less of a threat. He'd probably scratch at his neck in irritation. Dabi would be dismissive. He'd think fanfics are just a waste of time and not worthy of his attention. Toga, though, might be intrigued. If the fanfic had a lot of blood and gore in it, or if it focused on her relationships in an interesting way, she could be really into it.
In a My Hero Academia fanfiction, Gohan could be seen as a hero from another dimension. His moral code, which is similar to the heroes in MHA, would make him a natural fit. He could also train the students in new ways of combat, using his martial arts skills combined with his Saiyan abilities.
They may be either amused or annoyed. It depends on how they are portrayed in the fanfiction. If it's a positive and creative portrayal, they might be amused. But if it's a negative or inaccurate one, they could be annoyed.
Some characters might be intrigued. For example, Izuku Midoriya would probably be very interested in understanding how the Gamer Sparda powers work. He would see it as a new type of quirk to study and might try to find ways to incorporate the strategies used in the 'game' aspect into his own hero training.
Sasuke's powers could potentially be adapted in interesting ways in the MHA universe. His Sharingan could be seen as a unique quirk. For example, it could give him enhanced perception abilities similar to some quirks that allow for better combat prediction. He might be able to analyze an opponent's moves and fighting style just like in the Naruto world, but it would be framed within the context of MHA's hero - villain battles.
Goku's powers would be a game - changer in the MHA universe. His ability to fly would be unique among the heroes there. His Saiyan rage mode could be like an extreme version of a quirk awakening. But he might also have trouble fitting in at first. He's used to a more straightforward fight - or - be - fought world, while MHA has a lot of rules and regulations for heroes. However, his pure heart and desire to protect would surely make him a beloved hero.
Saitama's overpowered strength would likely be a huge disruptor in the MHA world. His simple yet extreme power of being able to defeat any enemy with one punch would make him a top - tier hero instantly. Genos, with his advanced cybernetic enhancements, would also be quite formidable. He could potentially be part of the development of new support items or hero gear in the MHA universe, given his knowledge of technology. And his combat abilities would put him in the ranks of the stronger heroes there.
Killua has a very stealthy side to him as well. In a My Hero Academia fanfiction, this could be manifested as a quirk that allows him to become invisible or at least very difficult to detect. He could use this in combination with his speed to surprise enemies. It would also be interesting to see how he would interact with other characters like Deku and Bakugo, perhaps teaching them some of his combat techniques.