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.
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 about Crowley going on a road trip across different dimensions could be really cool. He meets various versions of himself and other characters from different universes. Along the way, he discovers new things about the nature of good and evil and how his role fits into the grand scheme of things.
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.
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.
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.
Maybe Aziraphale is scared when a powerful otherworldly entity shows up that he thinks might harm Crowley. So he tries to protect Crowley in his own bumbling way. Crowley, in turn, is both amused and touched. He then shows Aziraphale that he can handle himself just fine, but also thanks Aziraphale for his concern. They end up having a little adventure trying to figure out what this new entity wants.
One plot idea could be that Crowley and Aziraphale try to hide Adam's true nature from the other celestial and infernal beings. They might pretend he's just an ordinary human boy while secretly teaching him about his powers. For example, they could take him on trips to historical and magical places to help him understand the balance between good and evil.
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.
One plot idea could be that Aziraphale and Crowley initially struggle with the moral implications of raising the Antichrist. They might try to guide him towards good, but the Antichrist's nature keeps pulling him in a more chaotic direction. For example, they could take him to various places to teach him about different cultures and values, but he ends up using that knowledge in unexpected and sometimes dangerous ways.
One common theme is Crowley's relationship with Aziraphale. A lot of fanfiction explores their friendship, the unspoken feelings between them, and how they navigate the world together. Another theme is Crowley's past in Hell, his experiences as a demon, and how that shapes his character in different ways than in the original work.