They could talk about Harry's parents. Sirius was Harry's father's best friend, so he could share many memories of James and Lily, like how they met at Hogwarts, their adventures in the Marauders' era, and what they were like as young wizards.
Another topic might be Harry's future. Sirius could give advice on how to deal with Voldemort, how to use his magic more effectively, and how to build relationships with other wizards in the fight against the Dark Lord. He might also encourage Harry to be true to himself and not let the prophecy define him completely.
Maybe Harry noticed Ginny acting strangely in the fanfiction and he wants Sirius' advice. Sirius has a lot of life experience and wisdom, so Harry might think he can help figure out what's going on with Ginny.
They might discuss Ginny's new-found magical abilities in the fanfiction. Maybe she has discovered a unique spell or has an extraordinary talent that's starting to surface.
Their talk could also enhance Harry's magical knowledge. Sirius could teach him spells or magical techniques that he hadn't learned at Hogwarts yet. This new knowledge would be very useful for Harry in his fight against the Dark Arts. Moreover, learning from Sirius would make Harry more independent in his magical growth as he gets advice from someone outside of the school environment.
Well, depending on the context of the fanfiction, Sirius could also be very understanding. If he knew that Harry and Hermione were just having a private conversation about important matters like their fight against Voldemort or schoolwork, he might just give them a knowing smile and say something like 'Carry on, then. Just making sure everything's all right.' He's a rather perceptive character, after all. And he trusts Harry a great deal, so he would likely give them the benefit of the doubt before assuming anything untoward.
She might talk about how much she worried during the war. It was a really tough time, and Ginny was surely concerned about Harry's safety all the time.
He could talk about his adventures outside of Hogwarts. For example, his encounters with Voldemort and how he overcame those challenges. This would inspire the students and give them a real - life example of facing great evil.
They could discuss the losses they endured during the battle. Remembering friends and family who didn't make it. Ginny might talk about how she felt when she thought she could lose Harry forever. Harry could then comfort her and say that they have to move forward in their lives while keeping the memories of the fallen alive.
It could be about Sirius teaching Harry advanced magic skills like Patronus charm. Sirius, being a skilled wizard, might train Harry to be better at dueling and facing dark wizards.
One possible plot twist could be that the marriage law is part of a dark plot. Maybe a powerful dark wizard has enacted it to create chaos. Harry and Sirius, instead of simply accepting it, start to investigate. They discover that the law is linked to an ancient prophecy that could bring either great good or great evil depending on how they handle their forced marriage situation.
The fanfiction might explore how Harry and Sirius' daughter overcome various obstacles. There could be issues like how the wizarding society views their relationship. Some might think it's inappropriate or too risky given Harry's status as the 'Chosen One'. But they would prove everyone wrong. They would build a life together, perhaps having children of their own who inherit their magical abilities. Their story could be one of love, adventure, and the triumph of the human (or wizard) spirit.