I also like 'Kurapika and Kuroro: A Shared Quest'. It's a story where they both embark on a journey to find a powerful and mysterious item in the Hunter x Hunter universe. Along the way, they face many challenges and their interactions are full of tension and some unexpected moments of camaraderie.
One common theme is power. Kuroro is a powerful character, and many fanfictions explore how he gains more power or uses his existing power in different situations.
A good fanfic in this regard often has well - developed characters. Since Kuroro and Kurapika are both complex, their motives, backstories, and personalities should be explored in depth. For example, Kurapika's quest for revenge against the Phantom Troupe and Kuroro's leadership of the troupe are key aspects.
Another interesting fanfiction is 'Kuroro and the Phantom Troupe Reunion'. It focuses on Kuroro's relationship with the members of the Phantom Troupe. The author does a great job of portraying their complex dynamics and the loyalty they have for each other.
Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
In the world of 'lucifer meets lucifer fanfiction', a common plot could involve a clash of egos. When two such powerful and complex characters meet, there's bound to be conflict. Some stories may also explore a romantic or at least a deeply emotional connection between them, as they are essentially the same being in different forms, and this connection can be both fascinating and disturbing at the same time.
I'm not entirely sure specifically what 'lucifer fiction' refers to. It could potentially be a genre of fictional works that somehow involve the concept of Lucifer, perhaps in a religious, mythological, or even a more creative and fictionalized context.