Romance is, of course, a very prevalent theme. Writers explore the relationship between Tatsumi and Chelsea, from the initial attraction to the development of deeper feelings. This could involve things like secret glances, stolen kisses, and declarations of love in various settings.
You can try fanfic websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search for 'Tatsumi x Chelsea' there. There are likely to be many different interpretations and storylines to choose from.
One common theme is friendship. Most fanfics explore how Chelsea and Austin's friendship develops over time. Another theme could be adventure. They often go on exciting adventures together in the stories.
One common theme is the exploration of their unspoken feelings. Since they are comrades in the original story, fanfictions often delve into how their relationship could evolve into something more romantic. Another theme is adventure, where they go on missions together and their relationship deepens during these dangerous outings.
A very common theme is self - discovery. Through their relationship with each other, Vaughn and Chelsea often discover new things about themselves. For example, they might realize their hidden strengths or weaknesses. Also, the theme of loyalty often pops up. They are loyal to each other in the face of various challenges and difficulties, which is a really heart - warming aspect of many of these fanfictions.
Since we should avoid the lemon aspect which is often related to adult content, in regular Tatsumi x Mine fanfiction, common themes could include their loyalty to each other. Given their story's context, themes of sacrifice and protecting one another might also be prevalent. They are both characters with strong personalities, so the exploration of how they handle their differences and grow closer could be another theme.
One common theme is friendship. Writers often explore how they become friends and support each other. Another could be adventure, where they go on quests together.
One common theme could be the idea of power and growth. Both Naruto and Tatsumi have their own paths of becoming stronger in their respective stories. So in fanfiction, it might be about how they help each other grow in power in a new situation. For example, Naruto teaching Tatsumi some ninja techniques and Tatsumi showing Naruto her own combat skills.
One common theme is the struggle between their duties and their love. Since they are both involved in a dangerous world, they often have to make tough choices that could affect their relationship.
One common theme is adventure. Since Tatsumi often gets involved with dragons, it leads to exciting adventures in different magical lands. Another theme is friendship. Tatsumi might form a strong bond with the dragons he meets, like a true friendship that helps them face various challenges together.
One common theme is self - discovery. When Tatsumi travels through time, he often learns more about himself, his strengths, and his weaknesses. For example, in some stories, he realizes that his past actions have had a greater impact than he thought.