A lot of MHA Kaminari x Jirou fanfictions focus on the idea of mutual understanding. They come from different backgrounds and have different quirks, but through getting to know each other better, they grow closer. There are also themes of jealousy, like when one of them might get a bit jealous if the other is spending too much time with someone else. Also, their shared experiences at UA, like going through tough battles together, are often used as a theme to bring them closer in the stories.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are a ton of MHA fanfictions there, and you can specifically search for Kaminari x Jirou ones. Another option is Fanfiction.net, though the quality can be a bit more variable. You could also check out Tumblr, as some users post links to their favorite fanfictions or even write short ones themselves.
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.
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.
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.
Another common theme is growth. The fanfictions may show how Ranboo and Tubbo grow as individuals while also growing their relationship. For example, they might learn to be more confident or face their past traumas, all while being in each other's company.
In some BNHA fanfics, Kaminari might be portrayed as the traitor, but it really depends on the specific plot and the writer's creativity. There are various interpretations in the fanfic world.