In this fanfiction, a possible plot could be that Sirius and Harry start a small magical garden together. Sirius uses his knowledge of magic plants, which he might have picked up during his time in Azkaban (maybe from observing the plants there in a strange way). Harry, on the other hand, discovers his own green thumb. Through this process, they bond over the care of the plants and Harry learns about patience and the cycle of life in the magical world. Another aspect could be Sirius trying to get Harry to be more confident in his magic. He does this by setting up small challenges for Harry around the house they live in, like making a Patronus strong enough to light up an entire room or levitating multiple heavy objects at once.
One idea could be Sirius teaching Harry all the secret passages around Hogwarts that he knew as a student. Another could be Sirius introducing Harry to the wizarding world's underground music scene. Or perhaps Sirius helping Harry deal with his nightmares about Voldemort by sharing his own experiences of facing dark magic.
A possible plot is that Sirius Black gets a tip - maybe from an unexpected source like a house - elf - that Harry Potter is being lured into a trap. Sirius races against time to reach Harry. When he does, Harry is in a wizarding duel with some powerful dark wizards. Sirius jumps into the fray, using his own powerful magic skills. He manages to deflect the dark spells aimed at Harry and then they both apparate away to safety. This could lead to a deeper bond between them and also new revelations about Sirius' past that help Harry in his fight against Voldemort.
One plot idea could be Hagrid teaching Harry all about the magical creatures in more detail. Since Hagrid has a great love for them, he could take Harry on secret trips to see rare ones. Another could be Hagrid helping Harry deal with the Dursleys in a more magical way, like using minor spells to make their lives a bit more difficult when they're mean to Harry. And perhaps Hagrid could be the one to first introduce Harry to the idea of different magical cultures, as he has so many connections in the wizarding world.
One plot idea could be that fem Harry Potter discovers a hidden prophecy that involves Sirius Black and they have to work together to figure out what it means. Along the way, they face challenges from dark wizards who also want to get their hands on the prophecy.
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.
One idea could be a humorous take on Sirius as a baby with his magical abilities already showing in unexpected ways while in diapers. For example, he might accidentally levitate his diaper when he gets excited.
Well, he could be in the hospital because of a curse. While there, he starts to form a bond with a young patient who has no family. Sirius then decides to protect this child and in the process, uncovers a plot against the wizarding world that has some connection to the hospital. Or perhaps Sirius is in a coma and his friends use magic to enter his mind in the hospital. They find out things about him they never knew before.
One interesting plot idea could be Harry growing up with Sirius's wild and rebellious spirit. He might be more daring in his adventures at Hogwarts, perhaps even forming a secret group earlier to oppose the Dark Arts in a more proactive way. Another could be the exploration of how Sirius's family history and his status as a Black influence Harry's view of pure - blood ideology. He could struggle with the expectations that come with being a Black heir while also trying to uphold the values he learned from his friends and Dumbledore.
Perhaps Sirius Black's daughter has a special connection to Harry through a long - forgotten prophecy. They start to notice strange coincidences and shared dreams. As they investigate, they find out that their destinies are intertwined in a way that could change the wizarding world. It could involve a journey to find a lost magical artifact that holds the key to fulfilling the prophecy.
One plot idea could be Sirius escaping Azkaban earlier and secretly taking care of baby Harry. He could teach Harry magic in hiding, away from the Dursleys. Another could be Sirius using his animagus form to be close to baby Harry without being detected, like a secret guardian watching over him at all times.
One idea is that a dark wizard targets Remus Lupin and Sirius Black because of their close relationship with Harry Potter. The fanfiction could focus on how they protect themselves and Harry, and how this situation brings out new aspects of their magical abilities and their bond with each other. They might have to seek help from some unexpected allies within the wizarding world to survive.