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.
The idea of them protecting each other is also common. For example, Crowley saving Aziraphale from some angelic danger or vice versa. And then there's the theme of self - discovery. They both have been part of their own 'worlds' for so long, and in fanfics, they often discover new things about themselves through their relationship with the other. It's really interesting to see how different authors play with these themes.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you can easily search for Good Omens Aziraphale x Crowley ones by using the relevant tags. There are also fanfic communities on Tumblr where fans might share their works or recommend good ones.
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.
A popular one is 'The Arrangement Expanded' which delves deeper into their early days of the arrangement. It shows how they started to trust each other more and more over time, with some really sweet and funny interactions.
A fanfiction could explore what would happen if Crowley and Aziraphale were accidentally swapped into each other's bodies. They would have to navigate the other's life and responsibilities, which would lead to some hilarious and also some very heart - warming moments as they understand each other better.
On Archive of Our Own, there are many great non - NSFW fanfics about aziraphale and crowley. You can search by tags like 'Good Omens', 'aziraphale/crowley', and filter for the appropriate rating. Some popular ones often explore their relationship in different scenarios, like dealing with a new threat in London while still maintaining their unique dynamic.
First, really understand the characters. Aziraphale is an angel but he has his own insecurities and fears. Crowley, as a demon, is more used to the darker side of things but cares deeply for Aziraphale. When writing Aziraphale being scared, show his internal struggle. For example, if he's scared of a threat to Earth, he might be torn between his angelic duty and his fear. Crowley's reaction should be in - character too. He might be sarcastic on the surface but very protective underneath. Don't forget to add some unique settings, like the little cafes or old churches they frequent.
There's also 'Crowley's Perspective'. It offers a unique take on their relationship by focusing mainly on Crowley's point of view. You get to see his internal thoughts and feelings about Aziraphale, which adds a whole new layer of depth to their relationship.
It's hard to say if they are extremely popular in a general sense. While there is a large and diverse fan community for Good Omens, not everyone is interested in this particular type of fanfic. Some fans might be more into the romantic or adventure - based fanfics about aziraphale and crowley.