This kind of fanfic is inappropriate. It objectifies the characters and may introduce negative and inappropriate ideas into the fan community. Fanfic should be more about exploring positive aspects of the characters, like their relationships in a non - sexual and respectful way.
First, focus on the positive traits of the characters. For example, their heroism or their unique abilities. Second, consider their relationships in a platonic way, like friendship or mentorship. Third, follow the general ethical guidelines for writing fanfic, which is to respect the original work and its intended audience.
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.
Lucifer can be a good comic depending on your preferences. If you enjoy dark and mythical themes, it's likely to appeal to you. The character development is strong, and the world-building is immersive.