The girl OC could be a noble from a foreign land who comes to England and crosses paths with Ciel and Sebastian. She has her own motives for being there, perhaps to find something that was lost from her family. She ends up getting involved with the Phantomhive's business, and Sebastian has to protect her while also trying to figure out her true intentions. There could be a lot of drama as she tries to navigate the dangerous world of the Black Butler universe, and she might also develop a romantic interest in either Ciel or Sebastian, which would add more complexity to the story.
One possible plotline could be that the girl is a new servant in the Black Butler household. While exploring the garden, she stumbles upon a strange object or a hidden door that leads her to a part of the Black Butler world she didn't know existed. This discovery could put her in danger as she uncovers secrets that some characters want to keep hidden.
One possible plot twist could be that the girl, instead of being severely injured, discovers she has some sort of supernatural power as a result of the accident. Maybe she can now see the spirits of the dead more clearly, which ties into the dark and supernatural themes often present in 'Black Butler'.
Perhaps she is a magical being. In the 'Black Butler' universe, there are all sorts of supernatural elements. She could be a fairy or a nymph who was trapped in the garden. Sebastian and Ciel might initially be suspicious of her but then find out she can be an ally in their battles against other dark forces.
One idea could be an OC who is a detective from another country coming to London to solve a series of mysterious murders that seem to be linked to the supernatural activities in the Black Butler world. This OC could cross paths with Sebastian and Ciel as they are also investigating the same case from their own motives.
Well, perhaps the snake could be a shape - shifting demon in the Black Butler universe. It falls in love with a human character, maybe a maid in the Phantomhive household. They meet when the snake is slithering around the manor gardens and the maid shows kindness to it, not knowing its true form. Their love story could be full of secrecy and challenges as they try to navigate the complex world of demons and humans in the Black Butler world.
A crossover could also have Loki from Marvel meeting the demons in Black Butler. Loki might try to manipulate them for his own gain, but the demons, especially Sebastian, are not easily fooled. This could lead to a power struggle between Loki's magic and the demons' otherworldly powers.
One possible plot development could be that the corset is used as a clue. For instance, it might have a hidden pocket or mark that leads to a mystery. Another is that a character's reaction to the corset - wearing or the corset - lacing process reveals their true feelings towards someone. Maybe they blush or become flustered, indicating an unspoken attraction.
A possible plot could be that Grell's sister has some sort of special power that she's been hiding. In the fanfiction, she could be forced to reveal it to save Grell from a dangerous situation in the Black Butler world. Maybe she has the power to manipulate time briefly, and she uses it to reverse a fatal attack on Grell.
First, establish a strong character for the human girl. Give her unique qualities and a clear goal. For example, she could be on a quest to save her family from the dark forces in the Black Butler world. Then, introduce the butler in an interesting way, highlighting his mysterious nature. Their initial encounter should be memorable.
In 'Black Butler OC Reincarnation Fanfic', a prevalent theme is power. The OC may discover new powers or have to deal with the power dynamics in the Black Butler world. For example, the power that Sebastian holds over others. Also, the theme of love and friendship is common. The OC might form new relationships, either romantic or platonic, with the characters in the Black Butler universe. They could fall in love with a character or develop a strong friendship that helps them in their new life. Additionally, the theme of survival is there as the world of Black Butler can be a dangerous place for a reincarnated OC.