One common plotline could be Sirius teaching Harry more about his family's history in a way Dumbledore didn't. For example, Sirius might share hidden secrets about the Potter family magic that Harry was never aware of.
Some fanfictions explore the idea of Sirius Black and Harry going on secret adventures together. Maybe they discover a hidden part of the wizarding world or try to solve an ancient mystery that has been long forgotten. This kind of plotline often involves a lot of action and exploration.
One interesting plotline could be Sirius teaching Harry advanced magic in secret. For example, Sirius might show Harry how to perform Patronus charms earlier than he does in the original story. This would give Harry an edge in dealing with Dementors and other dark creatures.
One common plotline is Sirius Black escaping from Azkaban earlier and secretly training Harry in advanced magic. Another could be Sirius and Harry going on a hidden adventure to uncover more about Harry's parents. There are also stories where Sirius plays a more active role in the fight against Voldemort at Hogwarts, using his knowledge of the Dark Arts and his Animagus form to their advantage.
One interesting plotline could be a mystery - solving one. Harry discovers some strange artifacts in Grimmauld Place that were left by Regulus. With Sirius (if he's alive in the fic), they try to figure out what these artifacts mean and it leads them to uncover a long - forgotten plot by Voldemort. Another plotline could be a redemption arc for Regulus. Harry, after learning about Regulus's attempts to undermine Voldemort, tries to clear his name in the wizarding world, which causes some friction with those who still see the Black family as mostly dark wizards. And there could be a plotline about family secrets. Sirius and Harry find out that there are hidden magical abilities in the Black family bloodline that only Regulus knew about, and they have to figure out how to use or control them.
One interesting plotline could be Fem Harry learning magic in a more unorthodox way under Sirius's guidance. Since Sirius is a bit of a rebel, he might teach her some spells that are not typically part of the Hogwarts curriculum early on. Another plotline could involve Fem Harry dealing with the challenges of being a female in the wizarding world, which has its own set of expectations and biases, all while having Sirius as her main support system. And perhaps there could be a plot where they go on secret adventures together, exploring parts of the wizarding world that are off - limits or forgotten.
One interesting plotline could be how Sirius helps Harry discover more about his parents in a more personal way. Since Sirius was James Potter's best friend, he could share countless stories and memories that Harry never knew. Another plotline might involve Sirius teaching Harry some advanced magic that he wouldn't learn at Hogwarts. This could give Harry an edge in dealing with the various threats he faces. Also, their relationship could be explored in depth, showing how Sirius becomes not just a guardian but also a true father figure to Harry.
One interesting plot could be the discovery of new magical creatures in the breeding grounds. Maybe Harry stumbles upon a rare species that has been thought extinct. Another could be the breeding grounds being in danger and Harry has to use his skills to protect it.
One interesting fanfiction could be about Sirius Black secretly training Harry Potter in advanced magic during his time at Hogwarts. This would add a whole new dimension to their relationship and Harry's growth as a wizard.
One common plotline is the daughter inheriting Harry's magical abilities in a unique way. For example, she might be able to communicate with magical creatures on a deeper level than Harry ever could. Another plot could be about her facing a new dark threat at Hogwarts, using her father's fame both as an advantage and a burden. She could also go on a journey to discover a long - lost magical artifact that Harry was somehow connected to in his past adventures.
In some fanfictions, the plotline could be that Draco and Harry adopt a child who is in danger from a dark wizard. They then have to go on the run, protecting the child and each other. It's a story of adventure and family. Also, there could be a story where they adopt a child who has a connection to an ancient magic, and they must figure out how to help the child control it while also dealing with their own relationship. Or perhaps they adopt a child who is a werewolf cub, and they face the stigma within the wizarding community while trying to raise the child with love.