It could be Draco Malfoy. In some fanfictions, they put aside their differences and develop a relationship. Sirius would be very surprised as he used to see Draco as an enemy. But seeing Draco's growth and his love for Harry could change Sirius' view.
Well, Sirius might see some of his own qualities in Harry's boyfriend. Maybe the boyfriend is brave like Sirius was, and Sirius would then start to warm up to him. He could share some of his old Hogwarts stories with the boyfriend, which would help the boyfriend understand Harry's world better.
In many fanfictions where Sirius meets Remus' parents, it can be a very interesting scenario. It might be filled with tension as Sirius is often seen as a bit of a rebel. Maybe at first, Remus' parents are cautious of Sirius, given his family's reputation. But as they get to know him, they could see his true self and how much he cares for Remus.
It could be Bellatrix Lestrange. Since she is a Death Eater and has a personal vendetta against Sirius, she would be a great antagonist. She might try to kill Sirius again and also target Harry to please Voldemort.
One common theme is protection. Sirius feels an overwhelming need to protect baby Harry, given what has happened to his parents. Another theme is the idea of family. Even though Sirius isn't Harry's father, the bond between them is portrayed as very strong, like a family bond. And there's often a theme of hope as well, that with Sirius in Harry's life, there's a glimmer of a better future for Harry.
Yes, there are likely to be such fanfictions in the vast world of fanfiction. Many fans like to create alternate family relationships and storylines for their favorite characters.
Yes, there are likely some fanfictions with this concept. Fanfiction writers often come up with creative and alternative family relationships for characters. You can search on popular fanfiction websites like Archive of Our Own or Fanfiction.net using relevant keywords to find such stories.
In a 'Harry Potter meets Sirius Black fanfiction', it could be interesting if Sirius and Harry found a way to time - travel. They could go back to the time when James and Lily were alive. This would not only allow Harry to meet his parents but also give Sirius a chance to change some events that led to his downfall. They might also interfere with the actions of Voldemort in the past, creating a whole new timeline and a different future for the wizarding world.
If it's a fanfiction about Class 1A meeting Bakugou's boyfriend, there could be a lot of comical moments. Maybe Mineta would try to be inappropriate and get scolded by Bakugou. The more responsible students like Iida might give Bakugou relationship advice. His boyfriend could be from another class or even another school. If he's from another school, Class 1A would be curious about the differences in their hero training. And Bakugou would have to balance his relationship with his training and his place in Class 1A.