Well, there could be a fanfic where Lucifer forms an unexpected alliance with an exorcist. This alliance could be born out of a mutual understanding of a greater threat that is looming over both the demon and human worlds. They have to put aside their differences and work together, which makes for a really exciting story.
You can try fanfiction.net. It has a large collection of fanfics from various fandoms, including 'ao no exorcist', so there's a good chance of finding some great lucifer - related fanfics there.
I really like the fanfic 'Rin and Shiemi: A New Chapter'. It starts off with them going on a mission together. In this story, they have to rely on each other more than ever. It has some really cute moments where Rin tries to protect Shiemi in his own clumsy way, and Shiemi in turn helps Rin understand his powers better.
One common theme is the clash of different power structures. In 'ao no exorcist', there are exorcist powers and demon - slaying swords, while in 'kuroshitsuji' there are demon contracts. When these two worlds cross, seeing how the characters with different power sources interact is a big theme.
One popular 'Ao no Exorcist Shura x Yukio' fanfic is 'Bond of Shadows'. It delves deep into their complex relationship, exploring how Shura's tough exterior softens around Yukio. Another great one is 'Unspoken Ties' which focuses on their shared experiences in the exorcist world and how it brings them closer. There's also 'A Different Kind of Bond' that shows their relationship evolving in a unique way as they face various challenges together.
One common theme is the growth of their relationship. It often starts as a friendship and gradually develops into something more. Another theme is the acceptance of Rin's true nature. Shiemi is often there to support Rin as he comes to terms with his demon heritage.
The relationship between Shura and Yukio is full of potential for interesting stories. Shura is a strong and experienced character, while Yukio is more reserved and complex. Their different personalities can create a lot of drama and tension in fanfics. For example, in many fanfics, Shura's boldness clashes with Yukio's more cautious nature, which makes for engaging reading.
I've also heard of 'Merged Realms'. It focuses on the cultural differences between the two universes. The more religious - based world of 'ao no exorcist' and the Victorian - era influenced 'kuroshitsuji' world clash and blend in interesting ways. The fanfic delves into how the characters adapt to these differences while also dealing with the overarching plot.
The Ao no Exorcist light novel is interesting in terms of its exploration of moral and ethical issues. As exorcists fight demons, they often have to make difficult decisions. Is it okay to kill a demon that was once a human? These kinds of questions are raised throughout the story, adding depth to the overall narrative.