One possible storyline could be Harry's struggle with his newfound loneliness and how it affects his magic. He might start to isolate himself more from his friends at Hogwarts, fearing that he'll lose them too. Another could be Harry's determination to avenge Sirius by seeking out the Death Eaters responsible for his death more aggressively.
A storyline could involve Harry trying to connect more with the Order of the Phoenix in a deeper way. Since Sirius was a part of it, Harry feels a stronger need to be involved. He could start training harder with the members, learning new spells and combat techniques. Meanwhile, he's also trying to come to terms with his own anger towards Dumbledore for not doing more to protect Sirius. This internal conflict and his external growth within the Order could be a central part of the fanfiction.
He becomes more withdrawn. Harry was always a bit guarded, but now he's even more so. He doesn't open up as easily to new people or even his old friends sometimes. He's also more determined. The loss of Sirius makes him more resolute in his fight against Voldemort, as if he has a personal vendetta now more than ever.
Maybe some of the old grudges between wizarding families that were thought to be resolved start to flare up again. This could lead to a lot of political intrigue within the wizarding community. And the children of the main characters could get caught up in this web of family feuds, having to decide whether to follow their family's old ways or try to make peace.
In some fanfictions, Harry becomes more determined to fight against Voldemort. He channels his grief into training harder and becoming a stronger wizard. He often has flashbacks of Sirius which fuel his anger towards the Death Eaters and Voldemort, seeing them as the root cause of his loss.
One possible storyline could be that Harry leaves to explore the wizarding world outside of Britain. He might go on a journey to discover new magical creatures and spells in far - off lands. Another could be that he leaves to find a place of peace and solitude, perhaps in a hidden magical enclave, to deal with the trauma of the war. And he could also leave to train young wizards in secret, away from the public eye, to ensure they are well - prepared for any future threats.
One possible storyline could be them rebuilding the wizarding world together. They might focus on restoring Hogwarts, with Ginny using her Quidditch skills to start a new team there and Harry helping with the overall magical education revival. Harry could also be involved in Auror training reform, and Ginny would support him through it all, and they'd start a family in the newly peaceful wizarding world.
One possible storyline could be Harry struggling with PTSD from the war. He might isolate himself in Grimmauld Place, haunted by the memories of those he lost. Another could be him trying to find a new purpose in life, perhaps traveling the wizarding world alone to discover forgotten magic. Or he could be dealing with the public's perception of him as the 'Chosen One' and feeling the pressure to always be strong and heroic.
One possible storyline could be the chaos that ensues during the wedding preparations. Maybe the venue has some last - minute issues, like the decorations not being set up correctly or the catering going wrong. Another could be the discovery of a long - lost relative who shows up and brings some unexpected family drama. For example, this relative might have a claim to a family heirloom that was supposed to be part of the wedding.
Ron and Hermione become even more protective of Harry. They are constantly by his side, trying to cheer him up. Their relationship with Harry deepens as they share his pain. In some fanfictions, this brings Ron and Hermione closer together romantically as well, as they bond over their shared concern for Harry.
Neville Longbottom could also play a more prominent role. Neville has his own growth arc in the series, and after Sirius's death, he could bond more with Harry over their shared experiences of loss. They could support each other in their fight against the Dark Arts. And perhaps some of the Death Eaters who were involved in Sirius's death could be main characters from the antagonist side, as Harry tries to track them down and bring them to justice.
One possible scenario could be in a battle against dark wizards. Maybe he's outnumbered and despite his best efforts, he succumbs to the dark magic. Another could be a self - sacrificing act to save his loved ones, like his family or friends. He might put himself in harm's way to protect them from a powerful curse.