A different scenario could be that the reader is an artist who is painting a mural about the end of the world. Crowley, seeing the mural, is fascinated and starts to talk to the reader. As they interact, the reader sees past Crowley's demon exterior and they form a deep connection. The story could then explore how they navigate their relationship in a world where angels and demons are not supposed to get along.
One possible fanfiction could be about the reader being a human who accidentally gets caught in the middle of the angel - demon conflict. Crowley, being the demon with a soft spot, decides to protect the reader. This could lead to a slow - burn romance as they face various challenges together.
The fanfiction could start with Akainu being injured and the reader finding him. Despite knowing who he is, the reader decides to help him. This act of kindness plants the seed for their relationship as Akainu is not used to such selfless acts towards him.
First, understand the character of Crowley. Know his traits like his love for his car, his somewhat cynical view of the world but also his underlying good heart. Then, create a believable reader character. Decide if they're going to be a strong - willed person or more of a shy type.
One popular type often features Crowley and the reader in various adventures through London, similar to the settings in the show. For example, there are fanfictions where the reader is a new resident in London and accidentally stumbles upon Crowley's world.
One common theme is protection. Crowley, being a demon with a complex personality, often ends up protecting the child reader from various threats in these fanfics. It could be other supernatural beings or just the general chaos of the world.
One great fanfic could be about Crowley taking the reader on a tour of his favorite hidden spots in London. It shows his softer side as he shares parts of his world with the reader.
As the relationship progresses, Crowley may become more protective and nurturing. He goes from being someone who only looks out for himself to someone who is willing to put the child's well - being above his own comfort. This development not only changes his relationship with the child but also how he views the world around him. He starts to see the world through the child's eyes, which is full of wonder and curiosity, and this makes him more empathetic towards others as well.
One heartwarming scene could be when Crowley protects the child reader from some bullies. He might use his demonic powers in a very subtle way, like making the bullies' shoelaces tie together so they can't chase the child. It shows his protective side towards the innocent child reader.
One common theme is the exploration of their relationship outside of the constraints of Heaven and Hell. Since they are an angel and a demon, their relationship is unique and fanfictions often play with how they can be together despite their opposing natures.
Archive of Our Own (AO3) is a great place to start. It has a large collection of Good Omens fanfics, including many focused on Crowley x Reader. You can use the search function and filter for the relationship you're interested in.