In a fanfiction plot, Harry might teleport to Sirius in the middle of a battle. Sirius is surprised but quickly realizes they can turn the tide of the battle together. They start using their combined skills and knowledge. Harry could learn some advanced defensive magic from Sirius during this time. And as they fight side by side, their bond grows stronger, which has a significant impact on Harry's development as a wizard and his understanding of his family.
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.
A possible plot is that she gets into trouble when some dark wizards mistake her for a key to some powerful dark magic related to Harry and Sirius. So she has to rely on her friends and her own wits to escape and prove that she's not what they think. Also, she could be involved in a time - traveling adventure, where she meets her father and godfather at different points in their lives, which gives her a new perspective on their relationships and her own identity.
One possible plot idea could be that Sirius catches Harry and Hermione in the middle of a secret plan to save a fellow student from a dark magic threat. Sirius, initially thinking they're up to something dangerous, then realizes their noble intentions and decides to help them. Another idea is that Sirius catches them while they are exploring a hidden part of Hogwarts that is related to an old mystery about his family. He then shares his knowledge about it with them, leading to an exciting adventure.
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 a mystery - solving one. There's a strange magical occurrence in Hogwarts that is affecting both Gryffindors and Slytherins. Hermione starts investigating, and she enlists the help of Sirius (who could be a ghost or somehow back in a semi - corporeal form). Harry is naturally drawn to help Hermione, and Draco gets involved because it might affect his family's standing. As they dig deeper, they find out that it's an ancient curse set by a long - forgotten wizard, and they have to find a way to break it using their combined skills and knowledge.
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 idea could be that Sirius teaches Harry all about his family's history in a much more in - depth way than Dumbledore ever did. He might show Harry secret passages in the Potter family home. Another plot point could be Harry learning advanced magic earlier under Sirius' tutelage, like Patronus charm at a younger age.
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 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.