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.
A very popular theme is the 'alternate universe' or AU. In these fanfics, the Good Omens world is reimagined. For instance, they could be in a modern high school setting, where Aziraphale is the nerdy but kind student and Crowley is the cool rebel. Or they could be in a historical setting, like the Victorian era. These AUs allow for a whole new set of adventures and relationship dynamics for Aziraphale and Crowley.
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.
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.
A common theme is also the exploration of their angelic and demonic natures. How do these aspects of them interact when they are together? There are also a lot of fanfics that focus on their adventures together, whether it's solving some mystery or going on a journey. It's all about the two of them and their dynamic.
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.
This comic has a great balance between comedy and drama. The art style is very appealing, and it delves deeper into the characters' emotions and motivations. It also expands on some of the storylines hinted at in the original material.
It can be considered popular. There are a large number of fans who are interested in this pairing. The ambiguity in their relationship in the source material gives fans the incentive to write fanfics exploring their possible romantic relationship. These fanfics can be found on various fanfic platforms with a significant number of reads and positive feedback in many cases.
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.
They have a complex and somewhat ambiguous relationship. They are both on opposite sides but also have a mutual understanding and a strange sort of friendship.
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.
In fanfics, the 'weight' could refer to the significance or importance of Aziraphale and Crowley. It might be about how much they matter to the overall plot or to each other. For example, a fanfic could focus on how their relationship is the 'weight' that holds the story together during a time of great upheaval in the Good Omens universe.