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.
Itachi would probably be very calm and composed. He might see Sarada as a symbol of the new generation of ninjas in the village. He could be gentle with her, perhaps sensing her potential and her desire to grow as a ninja.
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.
Sakura would probably be initially terrified. She would try to use her medical ninjutsu skills to defend herself or at least slow Itachi down. If she realizes she can't overpower him physically, she might try to talk to him, asking him why he's doing this.
Sasuke could also be confused. He has always had a mix of emotions towards Itachi, including love, hate, and envy. Being kidnapped by him would add another layer of confusion. He might wonder if Itachi has some new plan or if he's finally going to reveal some long - hidden truth. So, he might hold back on his full - fledged resistance and wait to see what Itachi has to say.
Sakura would probably be angry at herself for being captured. She's a proud ninja. So she would start looking for weaknesses in Itachi's hold on her. Maybe she would try to engage him in conversation to find out his motives, all the while formulating a plan to break free. She could also try to send out some sort of signal to her comrades if possible.