No, it's not appropriate. Deku is a character with his own depth, goals, and story within the MHA universe. Fanfic should aim to explore his hero journey, his relationships in terms of mutual respect and support, not create content that focuses on inappropriate or disrespectful ideas like this.
I don't really have a list of specific 'mha deku x boobs fanfic' as it might lead to some inappropriate or objectifying content. Instead, if you like Deku, you could search for fanfics that focus on his growth as a hero, his friendships, and his challenges. For example, there are many fics that explore his relationship with All Might or his classmates in a positive and meaningful way.
One interesting crossover could be the magic system of Harry Potter being introduced to the MHA world. For example, a character from MHA might discover they have magical abilities like a Hogwarts student. Another could be the heroes from MHA having to face off against dark wizards from the Harry Potter universe. It would be a really cool mix of the two different power dynamics.
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.
The idea of a 'dirty fanfic' in this context is not something to be encouraged. Instead, if one is interested in the sans x chara relationship, they could write a story about how they might team up to solve a mystery in the game world. This would be a much more positive and family - friendly take on their relationship.
There are fanfictions where the main characters react to different versions of their powers. For example, Gon and Killua reacting to a more powerful or darker version of their nen abilities. This can lead to interesting character development as they grapple with what they could become. It also allows the author to explore different power - up scenarios in a unique way.
One interesting Spy x Family reaction fanfic could be about how the Forgers react to Anya getting a top grade in school. They might be overly excited and throw a big celebration. Another could be the neighbors' reaction to seeing Loid's spy skills in action by accident. Maybe they think he's just really good at parkour or something strange.
I think Harry Potter himself would be very powerful in this crossover. He has the experience of fighting dark wizards and his magic skills. If he was in the MHA world, he could use his magic to enhance his physical abilities like the characters with quirks. His ability to resist the Imperius Curse and his knowledge of defensive magic would give him an edge over many MHA villains.
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.
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.