Build a plot that makes sense in the Good Omens universe. It could start with a chance encounter, like the reader finding Crowley in a strange situation in a London alley. As you write, show the development of their relationship through dialogue and small interactions. For example, Crowley's sarcastic remarks and the reader's reactions to them can be a great way to build chemistry.
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.
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.
First, think about how he gets injured. It could be in a battle with demons or angels, or an accident in the human world. Then, consider how this injury affects his relationship with Aziraphale and other characters. For example, does he become more dependent on Aziraphale? Also, describe his pain and how he copes with it in a way that stays true to his character.
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.
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.
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.
You can try fanfiction websites like Archive of Our Own (AO3). There are often many talented writers who create stories about Crowley and a child reader. Just search for 'Good Omens Crowley x Child Reader' in the search bar, and you'll be presented with a variety of options.
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 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.
There's also 'Aziraphale's Bookshop' which is set mostly in Aziraphale's beloved bookshop. It explores how Crowley visits and the little moments they share there. It's a really sweet fanfiction that captures the essence of their relationship through the cozy setting of the bookshop. There are some tender scenes that will make you smile.