In fanfiction, many stories revolve around Lucifer's fall from grace in more detail. Writers explore what really happened and how God felt about it. There are also stories where Lucifer becomes a sort of anti - hero, fighting against other forces of evil while still being at odds with God. Some fanfictions also play with the idea of an alternate universe where God and Lucifer have a completely different relationship, like being equal partners or Lucifer being the one in control.
One popular storyline could be Lucifer being punished for his past deeds by the main characters in the 'Supernatural' universe. For example, Sam and Dean might use some powerful magic or a special weapon to confine Lucifer and make him suffer for all the chaos he has caused. They could send him to a desolate dimension where he has to endure constant pain as a form of punishment.
Sam Winchester is quite popular to pair with Lucifer. Their connection in the show, with Sam being one of Lucifer's vessels, makes for an interesting dynamic in fanfiction. Also, Castiel. The angel's moral compass and his power contrast well with Lucifer's nature.
One of the popular pairings is Michael/Lucifer themselves. Their complex relationship, being brothers and eternal rivals, lends itself well to shipping in fanfiction. Writers like to explore the dynamic of their love - hate relationship in a romantic or at least very intense emotional context.
Well, 'Jean Winchester's Encounter with Lucifer' is very popular. It has a lot of action scenes where Jean battles Lucifer's minions while trying to understand Lucifer's motives. Another popular one is 'Lucifer and Jean: A Forbidden Alliance'. It plays on the idea of the two characters having to team up against a greater evil, which is a classic plot device but done really well in this fanfic.
In fanfiction, God can be characterized as more fallible. He might be shown as having made mistakes in dealing with Lucifer. Lucifer, on the other hand, could be seen as more sympathetic. For instance, he might have been pushed to rebel because of God's actions.
Well, a lot of fanfictions focus on Gabriel and Castiel having to adapt to the human world together. They might have to learn about human emotions and customs, which often leads to some comical and heart - warming situations. There are also storylines where they are on a quest to find a lost artifact that could change the balance of power in Heaven and Hell.
Fanfiction often takes creative liberties. In the case of Lucifer and God, some stories might depict them as having a secret understanding all along. Lucifer's rebellion could have been part of a grander plan by God. This would completely change the dynamic we see in the show. It could also lead to a different kind of endgame where instead of the usual conflict, they are working towards a common goal that benefits all of creation.
A very common storyline is their growth as individuals while still being connected. Baam is constantly evolving in power and understanding of the Tower, and Androssi has her own journey of self - discovery. Fanfiction might explore how their paths cross during these personal growth phases. Maybe Baam's newfound abilities help Androssi in a difficult situation, or vice versa. This can lead to a deepening of their bond.
Often, she's portrayed as a sort of mediator. In many fanfictions, she tries to reason with Lucifer, to bring him back to the side of good or at least stop his rampage. This can lead to some really emotional scenes between the two siblings. Also, there are stories where she falls in love with one of the regular characters in the Supernatural universe, like Sam or Dean. This love story then becomes intertwined with the overarching plot of good vs. evil.
One interesting storyline could be the interaction between the angels and demons from Good Omens and Lucifer. Maybe the characters from Good Omens end up in Lucifer's LA and have to deal with his brand of chaos. It could lead to some really funny and thought - provoking situations as they compare their different views on good and evil.