Love and loyalty are very common themes. Celty and Shinra have a deep love for each other that is often the focus. Fanfics show how they are loyal to one another through various situations. Also, the theme of discovery is there. As Celty discovers more about the human world through Shinra, and Shinra discovers more about Celty's true self, these aspects are frequently written about in the fanfics.
A common theme is the exploration of their opposite personalities. Hibana is more reserved and Shinra is brash, so fics often play with how they interact with and complement each other.
Love and loyalty are often themes. Shinra is always very loyal to Celty, and their love story, which is quite unique given Celty's nature, is a central part. Also, the theme of discovery. Celty is constantly discovering more about the human world with Shinra by her side, and Shinra is learning about Celty's world as well.
Often, there are plotlines where they have to deal with misunderstandings between them. For instance, Arthur might misinterpret Shinra's actions, leading to a rift in their relationship that they then have to repair. There are also plotlines where they are pitted against a common enemy that forces them to work closely together and understand each other better, strengthening their bond.
A sense of loyalty is often a theme. In these fanfics, Shinra and Tamaki are shown to be loyal to each other. Whether it's facing external threats or internal doubts in their relationship, they stand by one another. And the theme of shared dreams can also be found, like they might have a common goal for the future.
A third plotline could be set in a post - FF7 world where Cloud and Tseng meet again after a long time. There are still some unresolved tensions between them. However, they realize that they need to put their differences aside to face a new threat to the world. As they work together again, old feelings and memories resurface, and they have to deal with their complex relationship.
The relationship between Celty and Shinra is complex and interesting in the original work. Fanfiction allows writers to explore it further. Their different natures, Celty being a Dullahan and Shinra a human, create a lot of potential for unique storylines.
One possibility is that it focuses on a love story between Lucy and Rufus. They start off as rivals, perhaps competing in a magic competition. But as they get to know each other better, they realize they have more in common than they thought and fall in love. This could involve them sharing their dreams and fears, and overcoming obstacles together to be with each other.
There's no way to tell for sure what the story Rufus writes is without more information. However, it might be a coming - of - age story where Rufus reflects on his growth and the lessons he has learned in life. It could also be a love story, either about his own romantic experiences or a fictional love story he has created. Or it could be a mystery story, with Rufus as the detective trying to solve a puzzling case.
Another option is to check out some Final Fantasy 7 - focused forums. There are often threads where fans recommend their favorite fanfictions. You could ask specifically for Cloud x Tseng fanfictions in those forums, and the community might be able to point you in the right direction.