Some characters might be amused. For example, Hiccup, being the open - minded and curious one, could find it interesting to see how fans imagine different scenarios. He might even get some new ideas for his own adventures.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
The characters in 'httyd' would likely have complex reactions to the future in the fanfiction. Hiccup, for example, has always been a forward - thinking character. He would likely be intrigued by the possibilities the future holds, whether it's in terms of dragon - human relations or new inventions. However, he would also be concerned about protecting his tribe and the dragons. Astrid would be more practical. She would assess the potential threats and benefits. The dragons themselves, such as Toothless, would react based on their bond with their human counterparts. If they sense that Hiccup is positive about the future, they might be more open to it, but if there are signs of danger, they would be on high alert.
Some characters might be amused. For example, Hiccup could find it interesting to see different interpretations of his adventures in HTTYD 2. He might laugh at the creative liberties taken in the fanfiction.
Some MHA characters might be flattered. For example, Midoriya would probably be really excited and take it as a sign of how much he's inspired people. He might even look for positive lessons in the fanfiction to improve himself.
If the MHA characters were to react to a Mastermind fanfiction, some would be more accepting than others. Characters like Ochaco might find it really exciting. She's a fan of the heroes herself, so seeing different takes in fanfiction would be like getting a new perspective on the MHA world. However, characters like Shoto, who are more reserved, might be a bit hesitant. He might need some time to process what he's reading. But overall, it would be a very interesting scenario as it would show how the characters view different interpretations of their world.
Some characters would be shocked. For example, if the fanfiction had a really dark and unexpected Mastermind plot, like a hero turning out to be the mastermind of all the villains' plans, characters like Tsuyu would be very taken aback. She'd probably question the motives behind such a plot. Then there are characters like Denki. He might be more into it if it had some cool action scenes in the fanfiction, regardless of how out - of - the - blue the Mastermind storyline was. And Nejire would probably be super curious about the emotional depth of the characters in the fanfiction.
They may laugh. If the fanfiction contains some humorous scenarios or dialogues that are well - written. For instance, if there are some comical interactions between the dragons and the riders that are different from what's in the movie.
Some characters might laugh at the creative and sometimes out - of - the - box ideas in the fanfiction. For example, if there's a fanfic where a dragon can talk in a very strange accent.
They might get angry if the fanfiction shows the heroes in a very positive light and the villains as weak or stupid. They have their pride and don't like to be made fun of or belittled, even in fan - made stories.