One common theme is family. Since they both have complex family backgrounds in the Shadowhunter world, their married life often involves dealing with family issues. For example, how their families interact with each other.
Love and commitment are common themes. Their marriage is a new chapter, and fanfictions often explore how they stay committed to each other in the face of various difficulties.
Friendship is a very common theme. They are both important characters in the Shadowhunter universe, and many fanfictions explore how they become friends and support each other.
One common theme could be betrayal. Alec might be in a situation where he has to betray Clary for some reason, like loyalty to his own group or family, which causes her harm.
A theme that also pops up is the discovery of new sides of each other. Clary and Alec each have their own personalities and secrets. Fanfictions often explore how they learn more about each other, which in turn strengthens their bond. For example, Alec might discover a more vulnerable side of Clary, and vice versa.
One common theme is the adjustment of their relationship. With the pregnancy, they need to redefine their roles and how they interact. Another theme is the support from their friends and family. In many fanfictions, their friends play an important part in helping them through this new chapter.
One common theme is friendship. Many fanfics explore the deepening of the friendship between Alec and other characters. For example, how Alec and Clary become closer friends over time and support each other through various difficulties.
I recall a fanfiction where Alec and Clary are married and they go on a journey to discover a new realm. It's called 'Beyond the Married Horizon'. It's an exciting read as it combines their relationship with elements of adventure and mystery.
Well, one common theme could be the exploration of their relationship in more intimate or romantic settings. Another might be the development of their characters within those scenarios.
A plot could revolve around their family. They might decide to adopt a young Shadowhunter in need. This would bring out their parental instincts and also show how they support each other in raising the child. They could also face opposition from some in the Shadowhunter community who don't think they are suitable parents, but they prove them wrong through their love and determination.
One popular fanfiction site to find such stories is Archive of Our Own (AO3). You can simply search for 'Clary and Alec married' on it, and it'll show you a variety of stories. Some might focus on their married life in a post - war Shadowhunter world, dealing with the challenges of balancing family and their duties as Shadowhunters.