One common theme is the exploration of cultural differences. Zuko is from the Fire Nation with its own traditions, and Jin is from the Earth Kingdom. Their relationship can be a vehicle to show how these two different cultures interact and understand each other.
Love and relationship building are common themes. Since they come from different backgrounds in the Avatar universe, exploring how they bridge the gap between them is often seen. Another theme is personal growth, especially for Zuko as Jin can be a catalyst for his change.
I'm not sure of specific Zuko x Jin fanfiction titles off - hand, but you could also try fanfiction.net. However, the quality can vary more on this site. Look for stories that have a high number of reviews, as those are often the better - written ones. Usually, the ones that explore their relationship in the context of the Avatar world, with Jin's outgoing nature and Zuko's complex personality, are quite interesting.
I really like 'Jin and Zuko: A New Path'. It takes an interesting twist on their story. It shows how Jin's kindness could potentially change Zuko's journey in a unique way. There are some really touching scenes between them in this fanfic.
There are several 'zuko x jin fanfic' worth checking out. For example, 'Under the Moonlight' where they share a special moment. 'The Red String of Fate: Zuko x Jin' is based on the concept of fate bringing them together. In this fanfic, their connection is presented as something that was meant to be from the start. 'Jin's Influence on Zuko' is also a good one as it shows how Jin's positive attitude impacts Zuko's rather brooding personality.
In the fanfiction, it could be that Jin discovers she's pregnant during a time of unrest in their land. Zuko and Jin then have to find a way to keep the pregnancy a secret from their enemies. As the story progresses, they might find unexpected allies who help them. Zuko could also be training to become a better fighter to protect his family, while Jin focuses on taking care of herself during the pregnancy. This could lead to a lot of character development for both of them.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of fanfictions, and there are likely many great ones about Jin and Zuko there. Just use the search function and type in their names.
One common plotline is that Sokka gets into a dangerous situation during a battle or while exploring a new place. Zuko, using his firebending skills and newfound sense of heroism, swoops in to save him. This often leads to a moment of realization for both of them about their feelings for each other.
One great 'zuko x katara fanfic' is 'The Balance Within'. It beautifully explores their relationship as they work together to bring peace to the four nations. The character development is really well - done in this fic.