One common theme is redemption. Often, Lucifer is portrayed as a character seeking some form of redemption, and Dean is either the catalyst or the one who has to come to terms with Lucifer's attempt at change.
One popular dean x lucifer fanfiction is 'The Bond Beyond'. It explores a unique relationship between the two characters where they are forced to team up against a common enemy. Their interactions are filled with tension and an underlying sense of understanding that makes the story really engaging.
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.
Dean is likely a character who has some sort of relationship with Harry within the context of the fanfiction. He could be a classmate or a fellow wizard. He might be someone who has certain qualities or achievements that make Harry jealous.
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.
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.