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.
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.
Some of their friends from the Fire Nation, like Mai, could be hesitant. She might worry about the implications for the Fire Nation's future and Zuko's role as a leader. However, as time passes and she sees how much Zuko loves Katara and their unborn child, she becomes more accepting. On the other hand, the Water Tribe elders could have mixed reactions. Some might see it as a good sign of unity, while others might be concerned about cultural differences.
If it's a Disney fanfiction based on a princess story, the female characters in MHA like Momo Yaoyorozu might relate to the princesses' grace and leadership. Meanwhile, the male characters could either be in awe or a bit dismissive, depending on their personalities. For example, Todoroki might be quietly respectful, while Mineta might make some inappropriate remarks (if we're being honest about his character).
MHA characters might also be a bit jealous of some of the superpowers or unique abilities in other anime fanfiction. In their world, quirks are relatively well - defined, but in some other anime, there are really out - of - this - world powers. They could be in awe of the creativity and at the same time, try to figure out how they could incorporate some of those ideas into their own fighting styles. Like, if they read a fanfiction about a character who can manipulate time, they might start thinking about how they could use their quirks in a time - related way.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
Aang would be very happy and supportive. He has always been a friend to both Zuko and Katara, and he would see their union and the upcoming baby as a positive step towards more harmony in the world. Sokka might be a bit overprotective at first, especially towards Katara. But eventually, he would come around and be excited about being an uncle. Toph would probably make some sarcastic but also very endearing comments about the whole situation.
Aang would feel a sense of urgency and his connection to Katara would make him even more determined. He would call upon the spirits if necessary to find her. Sokka would be full of fury and his strategic mind would be working overtime to come up with a plan. He would also be very protective of his sister's honor during this time. Toph would be steadfast in her support for the rescue. She might even create some distractions to help the others get closer to Katara. The Fire Nation soldiers who respect Zuko might be confused by his actions, while those who dislike him might see it as an opportunity to bring him down. And Katara would be using her waterbending skills to defend herself and try to escape at every opportunity.