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?
It shows his vulnerability. Usually, Lucifer is seen as this powerful, menacing figure. But when he cries in fanfiction, it reveals that he has emotions and can be hurt.
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.
Lucifer in this fanfiction could be depicted as a complex character. He's likely to be extremely powerful, with a commanding presence. His intelligence would be sharp, always strategizing and thinking several steps ahead.
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.
There is no canonical love story between Eve and Lucifer. Eve was created by God for Adam in the Garden of Eden. Lucifer, on the other hand, was an angel who fell from grace due to his pride. Their stories are on different tracks in the religious lore. But if we were to create a fictional story, perhaps Lucifer, with his charm and his disdain for the rules, might approach Eve. Eve, being curious and in a world where she has only known Adam, could be intrigued by Lucifer's different nature. But again, this is pure fiction and not based on the original religious stories.