One common theme is self - acceptance. The fanfictions may show Sirius coming to terms with his body image in a world that might have different ideals. For example, he could learn to love himself despite not fitting the typical 'fit and handsome' image often associated with him in the original series.
You can try searching on fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can simply type 'Chubby Sirius Black' in the search bar. There are likely to be many works there that explore different aspects of this concept.
Well, in these fanfictions, we see a more vulnerable side of Sirius. For example, in many of them, his time in Azkaban is explored in greater detail. We get to understand the true extent of the psychological damage it caused him, which makes his character more three - dimensional.
Remus Lupin is often present. He was one of Sirius' old friends and a fellow Marauder. Their friendship can be delved into deeper, especially considering the complex history between them. Dumbledore is another common character. He plays an important role in the wizarding world and his interactions with Sirius can add depth to the story.
One possible fanfiction could be about Sirius Black being in a situation where he has to face Voldemort alone. Maybe he discovers a secret weakness of Voldemort that no one else knows. Sirius, with his brave and reckless nature, decides to take advantage of this knowledge to try and take down the Dark Lord once and for all.
A fanfiction could center around Sirius Black's attempts to rescue someone from Voldemort's clutches. He uses his skills as an Animagus and his knowledge of the wizarding world's hidden paths to get close to Voldemort's lair.
Well, Sirius Black's scars in fanfiction can vary a lot. Some fanfictions might focus on the emotional scars that are not as visible but are just as deep. His scars could be used to show his connection to other characters too. Maybe a scar he has is linked to a shared experience with Remus Lupin, which then builds on their friendship in the story.
For a more angsty read, there's 'Sirius' Forbidden Love'. It deals with the difficulties of Sirius being in a gay relationship in the wizarding world. It shows the internal struggles he faces, as well as the external pressures, and how love still manages to find a way through all that.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
The theme of redemption also shows up. Since Sirius had a somewhat 'tainted' reputation in the eyes of the Black family, some fanfictions explore if Orion could have played a role in his redemption or if Sirius could have redeemed the family name in Orion's eyes.