Another theme is the power dynamics. Given Lucifer's status as a fallen angel and his powers, how does his relationship with his wife (the OC) affect or change the power balance? For example, does she gain some powers through their connection? Or does she help him control his more devilish tendencies?
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can simply search for 'Lucifer Morningstar wife OC' in the search bar. There are often filters you can use to sort by popularity, rating, etc.
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.
The story of Lucifer and Lilith's love is often a complex and somewhat unorthodox one in certain mythologies. In some interpretations, they are both figures associated with rebellion and independence. Their love could be seen as a union of two strong, non - conformist entities. They might be drawn to each other because they both oppose the traditional order. However, different mythologies and fictional works may present their relationship in various ways.
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 recommend I, the Dark Giant! The main character of this novel is called Li Mu. In the Otto Universe, Li Mu inherited Lucifer's power and became a kind person who was obviously dark. I hope you like this fairy's recommendation. Muah ~😗