Well, it could be a really wild and interesting concept. Sirius Black is from the Harry Potter universe, and the Joker is from DC Comics. In a fanfiction, Sirius could perhaps adopt the Joker's chaotic and unpredictable nature. He might start playing pranks at Hogwarts in a much more extreme and menacing way, like the Joker's antics in Gotham. Maybe he'd use his Animagus form in really unexpected and Joker - like ways to startle people.
One possibility is that it could explore Hermione's struggle with her new identity. Since the Joker is a chaotic and often evil character in the DC universe, Hermione might have an internal battle between her Hogwarts - taught values of good and the dark influence of her 'father'. For example, she could be conflicted when it comes to using magic for good or being drawn to more devious uses like the Joker would.
A Black Star and Tsubaki pregnancy fanfiction could involve a lot of emotional development. It might start with the initial shock and then progress to them preparing for the new life. For example, Black Star could become more protective and responsible, while Tsubaki might be the calm and nurturing one. Their friends could also have various reactions, some being excited and others worried about how this would change their dynamic as a team.
It could involve Naruto being transported to the Marvel universe. He would be this strange, new kind of being with his chakra powers. Black Widow would be intrigued by him, and they might start off as allies. As they work together, a romantic connection could develop over time as they learn about each other's pasts and what they've been through.
In this Pokemon Black City dangerous fanfiction, perhaps there is a new type of Pokemon that has emerged, one that is extremely powerful and difficult to control. This Pokemon could be causing chaos in the city, attacking buildings and people. The city's authorities might be helpless against it, and it's up to a group of young and brave trainers to figure out how to stop it. They would have to face not only the Pokemon but also the various obstacles and enemies within the city that don't want them to succeed.
The plot could involve Sirius Black coming back as a vengeful spirit. After his death, his soul is not at peace. He haunts the halls of Hogwarts, especially the areas that held bad memories for him. Students start to see his apparition, and strange things begin to happen. For example, portraits on the walls start to cry or rooms suddenly become freezing cold. The main character, a young student, decides to find out the truth behind these hauntings and has to face the terrifying Sirius Black's spirit.
It would likely create a lot of chaos. Sirius Black is already a bit of a wildcard in the Harry Potter world. If he becomes like the Joker, the sense of safety at Hogwarts would be completely shattered. Students would be constantly on edge, not knowing what he'll do next. The teachers would have a hard time controlling him, and it could even lead to changes in the power dynamics between different houses.
The tattoos could signify his rebellious nature. As for the rituals, they might be a way to connect with his ancestors. For instance, a tattoo of a constellation could be related to a family ritual of stargazing and drawing power from the stars.
One possible plot could be that Sirius Black discovers he has a baby unexpectedly. Maybe it was a result of a past relationship he had before his time in Azkaban. He then has to learn how to be a father while also dealing with the wizarding world's reaction to his new situation, as many might still be suspicious of him.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
The fanfiction could start with Sirius being magically transported to a new, more just courtroom. Dumbledore could be secretly working behind the scenes to ensure a fair trial. The trial could be full of twists and turns. For example, the dementors that were involved in his original imprisonment might be called as witnesses, but Sirius could use his knowledge of their weaknesses to discredit them. And in the end, he is proven innocent, much to the joy of his supporters and the shock of his enemies.