Bakugo might initially be in denial. He's always had a complex relationship with Izuku, and seeing him in such a state could shatter his world view. He might become more withdrawn or, on the contrary, be spurred into a violent rage against whoever was responsible.
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.
One possible consequence could be the extreme shock and grief among his friends and allies. They would likely be devastated and it could lead to a downward spiral in their morale. For example, his classmates like Ochaco might blame themselves or become extremely angry at the situation.
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.
Communication is key. In the fanfiction, they could start having study sessions together. As they talk about their hero theories and strategies, they also start sharing personal stories. Shouto might open up about his relationship with his father, and Izuku could talk about her journey to become a hero despite being a girl in a somewhat sexist society. This will make them understand each other better and bring them closer.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
One plot idea could be that female Izuku and Shouto are paired up for a special hero training mission. They start off not getting along well because of their different approaches to heroics. Shouto is more by - the - book while Izuku is more instinctive. But as they face various challenges during the mission, they begin to understand and respect each other's methods, and a romantic bond starts to form.
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.
Eventually, he might come to accept it in a grudging way. If Izuku is wearing the clothes for a good reason, like in a team - up situation where it's part of a strategy, Bakugou might tolerate it. But he'll still probably make some snarky comments about how Izuku better not ruin his clothes.