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?
One interesting plot twist could be that Harry's true nature is revealed during a crucial battle. Instead of using his normal wizarding spells, he taps into his Lucifer - given powers and turns the tide in a way that no one expected. But this also makes everyone around him question who he really is.
Well, one way could be by exploring the clash of two very different magical worlds. Harry would have to deal with the expectations of the wizarding world from his Potter side and the dark, otherworldly nature of being Lucifer's son. Maybe he'd have new and strange powers that would both terrify and amaze his friends at Hogwarts.
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.
I'm not sure specifically which fanfics there are about Charlie Morningstar being pregnant. You could try searching on fanfic platforms like Archive of Our Own. There might be various creative interpretations in such fanfics, perhaps exploring different storylines around her character and this new situation.
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 usually begins with his expulsion from heaven. From there, it can take many forms, depending on the specific interpretation. It might focus on his interactions with humans, his attempts to regain his place, or his exploration of his newfound freedom and the consequences that come with it.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.
In the manga, Deku experiences various ups and downs. He not only becomes stronger physically but also matures emotionally as he takes on greater responsibilities. There are also many unexpected plot twists that shape his journey.