One common theme could be friendship. Their bond might be explored in different ways, like how they support each other through difficult times.
Love and romance can also be a common theme. Writers might imagine a romantic relationship between them and write about their courtship and how they deal with relationship issues.
I'm not sure which specific 'chii y hideki fanfic' you might like. But you can start by looking on fanfiction platforms like Archive of Our Own. They usually have a wide variety of fanfics for different fandoms.
Since I'm not familiar with the specific 'alice y shun fanfic', common themes in fanfics in general could be love, friendship, adventure. Maybe in this case, it could be the relationship between Alice and Shun, like how they overcome difficulties together.
Love and friendship are common themes. Since Naruto and Sakura have a long - standing relationship in the original Naruto series, fanfics often explore how that relationship could develop into a romantic one. Another theme is growth, both as individuals and as a couple.
A common theme is the power of friendship. Despite their differences, Naruto and Satsuki often form a strong bond in these fanfics. Another theme is growth. They both grow as characters, Naruto might help Satsuki become more open - minded and Satsuki could assist Naruto in becoming stronger.
Love is a very common theme. Their love story is often explored in different ways, like how they overcome obstacles to be together. Another theme is sacrifice, as both characters in the original work are known for their willingness to sacrifice for each other.
Love and relationship development is a very common theme. Most fanfics focus on how Harry and Ginny's relationship grows, from initial attraction to deep love.
One common theme is loyalty. Given their status as Saiyans, loyalty to their race and to each other is often explored. Another theme could be survival, as they live in a rather harsh universe. For example, they might have to fight against powerful foes together to survive.
Another theme is the exploration of their individual growth within the relationship. For example, Naruto may grow in his understanding of relationships while Ino might grow in her confidence as they get closer. It's about how they change and become better versions of themselves because of each other. And sometimes, there's also the theme of overcoming obstacles together, like dealing with the opinions of their friends or the demands of the ninja world.
Friendship is a very common theme. The stories often explore how Clara and Marina support each other through thick and thin.
The theme of destiny often plays a role. Many fanfics portray Serena and Darien as being fated to be together. Their relationship is seen as something that was meant to be from the start. This gives an air of inevitability to their love story and adds an element of magic and wonder. There are also themes of redemption in some fanfics, where one of them has to atone for past mistakes and their relationship is part of that journey.