Bungou Stray Dogs x Harry Potter
So obviously, Dumbledore, although old and wise, isn't exactly a saint. He is a very good wizard on the right side, but nobody is perfect. As such, he enlists the help of Yokohama's very own Port Mafia to help protect his students during Harry's fifth year at Hogwarts.
The Ministry is cracking down on him and Harry even more, and he knows for a fact they will be sending a representative, so he needs the Port Mafia to help protect his students and watch over them in ways he cannot.
Mori puts Dazai and Chuuya on the job, because who better to protect and attack but Soukoku? Who better to show the Port Mafia's strength but the most powerful duo they have?
So Dazai Osamu and Nakahara Chuuya, age 15, go to Hogwarts undercover as transfer students to watch over and protect the people of Hogwarts.
Before that happens, though, Dumbledore introduces them to the Order of the Phoenix as Japanese transfer students who are seeking to expand their knowledge and learn the ways of different cultures. Molly, as expected, welcomes them in and coddles them. The other adults greet them, although Moody is paranoid as usual. The kids, meanwhile, either greet them neutrally or stare at them in suspicion. Soukoku can't do anything about that, but they do know that they will need to keep a low profile. Dazai, being the genius he is, masters the first to fifth year curriculum quickly, while Chuuya struggles. Dazai also manages to communicate in English flawlessly, while Chuuya often begins rambling in either Japanese or French because he has difficulties with English. When Harry comes, Dazai and Chuuya sit awkwardly as Harry begins yelling at his friends (or at least Chuuya does; he does not know Harry, and he does not understand what The Boy Who Lived is saying). Dazai pretends not to listen to their conversation as he begins scheming in his head using the information he just absorbed and all the other information he learn up until that point.
Chuuya and Dazai introduce themselves to Harry as the boy stares awkwardly at them.
Before the first day back at school starts, Soukoku are sorted so that they do not go through the hassle of being sorted with the first years.
Dazai is sorted into Slytherin (because although he fits in Ravenclaw he's waaaaay to cunning for that) and Chuuya is sorted into Gryffindor (although maybe I could put him in Hufflepuff? He's loyal but then again that's the only about him that fits with that house).
Although they canonically have a sort of rivalry and love-hate relationship with each other, Soukoku still consider the other as a partner and someone they are relatively close to. They understand each other better than others ever have, and they rely on each other due to their abilities. For Chuuya, Dazai makes the voice in his head (Arahabaki) go away. For Dazai, Chuuya makes him feel less empty and void. So although they have that love-hate relationship, they still are relatively close and somewhat dependent on each other. It's harder for them to do their job when they are in different houses, but they still manage to make time and converse. Due to this, their respective houses are often outraged at them, due to the large Slytherin-Gryffindor rivalry. Soukoku don't care about that though. They don't understand the point of it. They find it rather stupid, actually. Why would people have the time for that when there are so many other things happening in the world? Due to this, some teachers have hope that Hogwarts will learn to band together again, as it used to during the time of the Founders.
Dazai and Chuuya realize they have their work cut out for them with all the trouble Harry gets into. His first class in Defense Against the Dark Arts, and he's already got detention. They have no idea just how much they'll have to work just to keep Harry relatively safe and out of trouble. With all the things that happen in Harry's fifth year, such as his hatred for the Ministry (Umbridge especially), the entire thing with the thestrals, Hagrid, Dumbledore, the prophecy, etc. etc., they know it'll be incredibly difficult.
So they follow Harry when he breaks into the Ministry. They encounter Voldemort and his Death Eaters, and Soukoku tells the students to let them handle it.
Obviously the others argue, out of pride or what, who knows, but Luna says to just stay back and watch Soukoku do their work. Immediately, the students are in awe of Soukoku's teamwork and efficiency, of Dazai's tactics and cunning, of Chuuya's strength and power.
Soukoku begins to overpower them, and Voldemort and his Death Eaters don't like that. Before they could do any worse, though, Dumbledore arrives.
The Ministry barges in and see Voldemort, and people can no longer stay in denial about Voldemort's return.
That's just my relative plot idea, but I'm a generally lazy person who already struggles to write, it's a lot harder to do complicated crossover fics, so I don't think I'll ever get to this idea.