Midoriya would have a very enthusiastic reaction. He would be jotting down notes about the different ways the Spy x Family characters use their skills and how it relates to his concept of using quirks. He might be inspired to think about new ways to use his own quirk based on what he reads in the fanfiction. For instance, seeing Anya use her telepathy in creative ways might give him ideas for new combat strategies.
Tsuyu Asui might have a rather calm and observant reaction. She would notice the small details in the fanfiction, like how the Forger family adjusts to living together. She might compare it to how the students at UA live together in the dorms. Tsuyu would also probably be interested in the different personalities in Spy x Family and how they interact, much like she observes her classmates in MHA.
I think All Might's reaction would be interesting. He might see Loid as a kind of hero in a different sense. Loid's selfless dedication to his mission to prevent war, even if it means living a double life, could remind All Might of his own role as a symbol of peace. All Might would probably respect Loid's determination.
Midoriya Izuku could also have an interesting reaction. He's always trying to understand the villains better, and Dabi is a very mysterious villain. He might study the fanfictions to look for clues or insights into Dabi's character that he could use in a potential confrontation or in his quest to become a better hero. He might be surprised by some of the creative interpretations in the fanfictions.
I think Midoriya's reaction would be the most interesting. He's such a huge hero fan and analyst. He'd dissect every aspect of Batman in the fanfiction, from his combat skills to his psychological state. He might even start theorizing about how Batman could exist in the MHA world and what kind of Quirk he could have.
Some MHA characters might be really intrigued by Disney fanfiction. For example, Izuku Midoriya would probably be super excited. He loves stories and analyzing different characters' motives, just like he does with heroes. He'd be all about exploring the new worlds and character relationships in the Disney fanfics.
The MHA characters' reactions could vary widely. For instance, All Might might see the Naruto fanfiction as a great source of inspiration. He could admire the way the characters in Naruto strive for their goals, just like heroes in their own world. Todoroki might be a bit detached but still interested. He'd analyze how the power systems in Naruto compare to quirks in their world. Mineta would probably be more interested in any fan service - like elements if there were any, but that's not a very positive reaction. Overall, it really depends on each character's personality.
Some MHA characters might be really intrigued. For example, Izuku would probably study the magic systems in Fairy Tail fanfiction with great enthusiasm, seeing it as a new form of 'quirks' in a way. He'd be taking notes and trying to understand how the characters use their magic powers for good.
Bakugo Katsuki, on the other hand, might be more dismissive at first. He'd think it's all just some made - up nonsense that has nothing to do with real hero work. But if he found a fanfiction that had an interesting battle scenario or a strong - willed character, he might grudgingly admit it had some merit. He'd probably scoff at any overly emotional or sappy parts though.
Twilight is very interesting. As a spy, he has to balance his dangerous missions with his new role as a father in the family. His intelligence and ability to adapt to different situations are fascinating. He is constantly calculating and planning, but also starting to develop real emotions for his 'family' members.
All the characters in Spy x Family story have their own charm. But if I had to pick one more, it would be Bond, Anya's dog. He has some mysterious abilities as well. Bond is very loyal to Anya and often gets involved in the family's adventures. He adds another layer of fun and mystery to the already complex and interesting world of Spy x Family.
Ruby's adventurous spirit would make her excited about the Spy x Family plot. She'd be eager to see how the characters handle their dangerous situations. Weiss, being proud and proper, might have a harder time getting into it at first but her sense of justice would eventually draw her in. Blake, as a more introverted and secretive character, would likely understand the characters' hidden motives well. Yang's boldness would lead her to enjoy the action - heavy parts of the fanfiction.
Loid would also be an engaging character to center on. His spy skills and ability to adapt to any situation could be really cool in the Naruto world. He could use his disguise and infiltration skills to gather information about the ninja clans or even help Naruto on a mission. His calm and calculated nature would be a contrast to the more impulsive and emotional nature of some of the Naruto characters.