Another option is Wattpad. Although it has a more diverse range of content, you can still come across some interesting Sirius and Bellatrix love fanfictions. It might take a bit more searching compared to AO3, but it's worth a look.
There are some out there. However, it's important to note that Sirius and Bellatrix come from different sides in the 'Harry Potter' universe, so these fanfictions often involve complex character developments and reinterpretations. You might have to sift through a lot to find the really well - written ones.
There's also 'Twisted Fates'. This fanfiction plays with the idea of fate and how Sirius Black and Bellatrix Lestrange could have had a different kind of connection. It's a very creative piece that takes liberties with the characters but still stays true to their core traits. The story is full of unexpected twists and turns that keep you hooked from start to finish.
A lot of these fanfictions tend to explore the idea of redemption or corruption. For instance, Bellatrix might be portrayed as being 'corrupted' by her dark side, and Sirius could be seen as a potential 'redeemer' in some stories. And of course, the sexual tension and chemistry between them is a big part of the lemon aspect, often explored through their encounters. This can be a very complex and often controversial exploration in fanfiction writing.
There's a fanfiction that delves into their backstories. It shows how they were both part of the Black family but took such different paths. Sirius, with his more progressive views, and Bellatrix, with her dark and twisted loyalty to Voldemort. The story might have some interesting flashbacks and interactions that are both poignant and full of conflict.
One possible storyline could be that Bellatrix, despite her dark past, has a change of heart due to some magical influence or a deep - seated love for the Potter family. Sirius, of course, would be a positive influence. They might raise Harry in a more unorthodox way, teaching him both light and dark magic in a balanced way so he can understand the full spectrum. This could lead to Harry being a more powerful and empathetic wizard, able to bridge the gap between different magical factions.
There are many fanfiction platforms where you might find Sirius Bellatrix stories, like Fanfiction.net or Archive of Our Own. However, I can't specifically recommend a 'lemon' (which usually implies sexual content) fanfiction as it goes against the rules of many platforms. You can search for non - explicit stories with their complex relationship as the focus on these sites.
The theme of redemption can also be present. Some fanfictions might explore if there is any chance for Bellatrix to turn away from the Dark Side, or how Sirius's actions might have an impact on her in some way. This can create a complex and interesting character dynamic, especially considering Bellatrix's deep - seated loyalty to Voldemort.
Sirius's friends, like the Marauders, would be extremely shocked. They would probably be very concerned about Sirius as they know his feelings towards Bellatrix as an enemy. They might try to convince him to break away from the situation, thinking it's some sort of dark plot against him. James, in particular, would be very protective and would want to figure out what's really going on.
One common theme is the struggle between good and evil. Since Sirius is typically seen as a 'good' character and Bellatrix as 'evil', the fanfictions often explore how their relationship might change their natures. Another theme is family loyalty and betrayal. As members of the Black family, their union can bring out different reactions from family members.
The contrast between them. Sirius is a rebel with a good heart, while Bellatrix is a devoted Death Eater. Their opposing natures make for great drama in fanfiction. For example, in many stories, they are forced to interact due to circumstances, and it creates a lot of tension.