Well, Bakugo might scoff at first. He'd probably think Batman is too brooding and not as straightforward as he likes his heroes. But then as he reads more of the fanfiction, he might grudgingly admit that Batman's detective skills are cool. Deku, on the other hand, would be super interested. He'd be analyzing every move Batman makes in the fanfiction, taking notes in his head about Batman's strategies and how they could be applied in his own hero work. Uraraka would be more focused on the relationships in the fanfiction. She'd be all 'aww' at the cute moments and feel for the characters when they're in trouble.
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.
Love and romance are also prevalent. The female character might fall in love with Shoto as she gets to know his kind heart beneath his cold exterior. This can lead to a lot of sweet and sometimes angsty moments as they navigate their relationship.
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.