The theme of 'adventure together' is also quite common. Sebastian and the reader embark on some sort of journey, whether it's a physical exploration of a new world or a journey of self - discovery. This allows for a lot of exciting plot development and character interaction.
One popular plotline is the reader being a new addition to the Phantomhive household. This could lead to both Sebastian and Ciel being intrigued by the reader. Another is the reader having some special power or ability that draws Sebastian and Ciel towards them. For example, the reader might be able to see the supernatural more clearly than others, which interests Sebastian as a demon, and Ciel as someone who often deals with the occult in his work. A third plotline could be the reader being from a different world or time, and having to adapt to the Victorian era where Ciel and Sebastian exist, while also dealing with their growing feelings for the two.
The theme of anticipation is also quite prevalent. Both Sebastian and the reader are anticipating the arrival of the baby. This can be shown through things like preparing the nursery together, or just talking about what the baby will be like. There's a sense of excitement and nervousness that runs throughout these stories.
The theme of sacrifice also appears. Sebastian, as a yandere, might be willing to sacrifice anything or anyone who stands in the way of his relationship with the reader. This can lead to some very intense and dramatic plot points in the fanfiction.
One common theme is redemption. Sebastian is often portrayed as a bit of a bad boy in Glee, and in fanfiction, he might seek redemption through his relationship with the reader. Another theme is rivalry. Maybe there's a love triangle with another character from the show.
Another theme is the anticipation and preparation for the new arrival. The stories could focus on how they decorate the nursery, pick out baby names, or attend prenatal classes together. It's all about building up to the big moment of the baby's birth and how their relationship evolves during this process.
One common theme is the power dynamic. Sebastian is a powerful demon and the reader is often in a more vulnerable position, so exploring how that power is balanced or shifted is frequent. For example, in many fics, the reader might initially be at Sebastian's mercy but gradually gain some form of influence over him.
A very common theme is the forbidden love aspect. Given Tobirama's position or certain circumstances in the Naruto universe, a relationship with the reader might be seen as forbidden, which adds a lot of tension and drama to the story. Also, stories centered around Tobirama teaching the reader something, like ninja skills, are quite popular.
One popular theme is the exploration of Thrawn's intelligence. Fanfictions often show how the reader character is drawn to his brilliant mind. Another theme is the clash of cultures, as Thrawn comes from a different background. And of course, the theme of love or a deep connection between the two characters is very common.
One popular theme is power and its implications. Since Rhysand is a powerful high lord, exploring how that power affects his relationship with the reader is common. Another theme is redemption. Maybe the reader helps Rhysand overcome some of his past traumas. Love against all odds is also a big theme. It could be that they are from different worlds or have different loyalties but still fall in love.
One popular theme is the power dynamic between Sebastian and the reader. Since Sebastian is a demon with great power, it often leads to situations where the reader is in a vulnerable yet alluring position. Another theme could be the exploration of the supernatural world through their relationship. For example, how the reader gets involved in the dark and mysterious affairs that Sebastian is a part of.