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.
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.
In a fanfiction where Petunia meets Sirius, it could be a very interesting encounter. Sirius, being a wizard from the wizarding world, might shock Petunia at first. He could show her some simple magic, like making a flower bloom, which would both terrify and fascinate her.
Well, Sirius might be shocked by the Dursleys' treatment of Harry. He could try to rescue Harry from their mean ways.
One possibility is that their meeting is a vision or a dream - like encounter. Harry could be drawn into Azkaban in his sleep or through a magical object. During this encounter, Sirius could give Harry advice on how to deal with the upcoming challenges at Hogwarts. He might warn Harry about certain people who can't be trusted and offer his insights on the growing dark forces.
Perhaps Sirius will be overawed by Remus' parents' love for their son. He might compare it to his own family situation and feel a bit envious. On the other hand, Remus' parents might see Sirius as a broken soul in need of love and acceptance. This could lead to a very heart - warming relationship where they take Sirius under their wing and help him become a better person while also fully accepting his relationship with Remus.
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.
In the fanfiction, they might have a heartfelt reunion. They could share all the things they didn't get to say before. Maybe they'd talk about their friends left behind, like Remus and Harry. They'd probably also recall their adventures at Hogwarts, from the pranks they played to the battles they fought together.
In this fanfiction, perhaps James Sirius Potter could learn some of the classic pranks the Marauders were famous for. He might get into all sorts of adventures, following in their footsteps and causing a bit of chaos at Hogwarts just like they did.
They might start by choosing a color scheme. Maybe they'd go for Gryffindor colors like red and gold to show Harry's house pride. Sirius might use his magic to move the furniture around easily while Harry decides where to put his Quidditch memorabilia. They could also add some protective enchantments around the flat while they decorate, making it a safe and cozy place.
In a fanfiction scenario, James Sirius Potter could be a bit of a troublemaker. When McGonagall meets him, she would have to decide how to deal with him. She could choose to be lenient, seeing the potential in him, or she could be very firm. However, as she gets to know him better, she realizes that he has a unique charm and talent, much like his father. And she could start to encourage him to use his skills in a positive way, perhaps guiding him towards a particular area of magic study that she thinks would suit him.