Another theme is the power struggle. Michael and Lucifer are both powerful beings, and in fanfics, this often plays out. They might compete for power at first, but then realize that there are more important things than power, like their connection. It makes for a very engaging dynamic in the stories.
The theme of forbidden love is also quite prevalent. Since they are on different sides in the traditional lore, their love is seen as something that goes against the norm. Fanfics often play with this idea, showing how they overcome the obstacles of society (in the context of the story's universe) to be together.
The theme of trust is quite prevalent. Given Lucifer's reputation, the reader might struggle with trusting him at first. This can create a lot of tension and drama in the story. And there's also the theme of sacrifice. Either Lucifer or the reader may have to make sacrifices in order to be together or to achieve a certain goal in the story.
In Selene x Michael fanfics, the theme of adventure is quite common. They could be on a journey to save their world or a particular place that is important to them. Along the way, their relationship deepens. Also, the theme of redemption can be seen, where one or both of them have made mistakes in the past and their relationship helps them find a way to redeem themselves.
Another theme could be growth. They might show how Michael and Freddo grow as individuals while being in each other's lives. For example, they could learn important life lessons from each other. This theme allows for character development and makes the story more engaging.
Friendship is a very common theme. Most fanfics explore how Jeremy and Michael are there for each other, like in 'Jeremy and Michael: The True Friends' where they always have each other's backs.
One common theme could be the exploration of their relationship dynamics in a new light, perhaps emphasizing the emotional connection that might develop between them.
One common theme is friendship turning into love. Often, the reader starts as a friend or acquaintance of Michael and over time, their relationship deepens into something more romantic.
Another theme is redemption. Some fanfics explore whether the reader can somehow bring out a different side of Michael, like a more human side. For example, in some stories, the reader's presence might make Michael question his violent ways.
Adventure themes might also be present. For example, they could be on a quest together in a fictional world, and during that adventure, their relationship deepens. This adds an exciting element to the story.
Common themes include survival. Since Laurie is constantly trying to survive Michael's attacks in the original movies, fanfics might expand on this. Also, there's the theme of obsession. Michael is obsessed with Laurie in a way, and fanfics can explore this further, like what if Laurie becomes equally obsessed with stopping Michael or understanding his motives. There's also the theme of redemption in some cases, where maybe Michael could be redeemed through his connection with Laurie, though that's a very different take on the classic horror relationship.
A prevalent theme is also the idea of forbidden love. Lucifer being who he is, a relationship with him in the context of the 'Supernatural' universe is often seen as something that goes against the norms. There's also the theme of temptation. Lucifer is the tempter in the show, and in fanfics, he might be tempting the reader character in various ways, whether it's with power, knowledge, or something else. Fanfics may also delve into the idea of the reader character being able to see a different side of Lucifer that others don't, like a more vulnerable or misunderstood side.