A great plot twist could be that Ranmao is actually related to one of Ciel's enemies in some way. Maybe she was planted as a spy near him, but over time she has developed real feelings for Ciel and now has to choose between her loyalty to her original group and her newfound loyalty to Ciel.
How about a plot twist where Ciel loses his memory and Ranmao has to convince him that he is the Earl of Phantomhive? Along the way, she discovers new sides of Ciel that she never knew before, and Ciel starts to see Ranmao in a different light as well. This could lead to a shift in their relationship.
A fanfiction could explore a different timeline where Ranmao becomes Ciel's bodyguard instead of Sebastian. This would lead to a whole new dynamic as Ranmao has her own unique skills and personality compared to Sebastian. She might be more impulsive in protecting Ciel, which could lead to some interesting situations.
A possible plot twist could be that the flu Ciel has is not a normal illness but actually the result of a curse. This would add a supernatural element to the story and make Sebastian's task of curing him much more difficult as he has to not only deal with the physical symptoms but also break the curse.
Another theme is mystery. Given her somewhat enigmatic character in the original 'Black Butler', fanfictions often play up on this aspect. They might explore her past, where she came from, or what her true motives are. This adds an air of intrigue to the stories. For instance, some fanfictions might suggest she has a hidden agenda that gradually unfolds throughout the story.
One popular 'Black Butler Ranmao' fanfiction might be 'Ranmao's Hidden Adventures' where she goes on a secret mission in the Black Butler universe. It's full of action and shows a different side of her character.
One idea could be Female Ciel trying to navigate the underworld of the Black Butler universe while facing discrimination for being a female in a male - dominated world. She might have to prove herself even more than her male counterpart did.
One idea could be Ciel exploring his new demonic powers. He might start to see the world in a completely different way, with his values shifting as he is no longer bound by human limitations. For example, he could use his powers to seek revenge in a more extreme way than he could as a human.
One interesting plot idea could be a time - travel scenario. Sebastian and Ciel are sent back in time to a period in the Phantomhive family's history that is shrouded in mystery. They have to navigate the past, deal with their ancestors, and also face the fact that their actions in the past could change the present they came from. Another idea is a role - reversal. Ciel becomes the demon and Sebastian the human servant, which would completely turn their relationship dynamics on its head and lead to all sorts of comical and also thought - provoking situations. A third plot could be a crossover with another popular anime world, like they end up in the world of Naruto and have to adapt to the ninja way while still maintaining their own goals and relationship.
One idea could be that Claude discovers a hidden power within Ciel that even Ciel himself is unaware of. This power could be related to Ciel's past or family heritage. Another plot could be that Claude has to protect Ciel from a new and powerful enemy that is specifically targeting Ciel for some unknown reason. Maybe this enemy has a connection to Claude's past as well.
Perhaps a story where Elizabeth and Ciel time - travel back to a period before all the tragedy in Ciel's life occurred. Elizabeth could then try to prevent the events that led to Ciel's current state. This would involve her getting to know a more innocent Ciel and also dealing with the consequences of changing the past, such as how it might affect their relationship in the present.
One plot idea could be the reader being a new servant in the Phantomhive manor. Ciel is initially suspicious but as time passes, he starts to trust and rely on the reader in a unique way. Maybe the reader has a special skill that helps Ciel in his investigations or in dealing with the other nobles.