A good one often has well - written characterizations. Sebastian's personality should be in - line with the original anime or manga. The ticklish aspect should be incorporated in a way that feels natural to his character, perhaps as part of a light - hearted moment or a unique plot device.
Well, a fanfic centered around Claude and Sebastian's tickle moments might explore their personalities in a new way. Since they are both so serious most of the time, tickling could be a way to break down their usual stoic facades. It could show a more vulnerable side of them. Perhaps in the fanfic, there's a magical or accidental situation that causes them to be ticklish, and they have to deal with this unexpected and rather embarrassing situation in their own unique ways.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often many fanfics related to 'Black Butler' there, and by using the search terms 'Sebastian' and 'ticklish', you might be able to find what you're looking for.
One popular theme is Sebastian's relationship with Ciel in different AU (alternate universe) settings. For example, in a modern AU, how they adapt to a world without the traditional master - butler dynamic. Another theme is exploring Sebastian's past as a demon, like his experiences before making the contract with Ciel. And some fanfics focus on Sebastian's interactions with other characters from the 'Black Butler' universe in unexpected ways, such as teaming up with Grell for a particular task.
In many Black Butler fanfics, the relationship between Sebastian and Grell is complex. Grell has an unorthodox infatuation with Sebastian. Sebastian, on the other hand, often sees Grell as an annoyance or a tool depending on the plot of the fanfic. Sometimes, fanfic authors play with their dynamic in a humorous way, like Grell's over - the - top attempts to get Sebastian's attention while Sebastian just wants to complete his tasks for Ciel.
There could be a lot of hilarity. Maybe they are in the middle of a demon - related task and suddenly, a magical spell makes them ticklish. Sebastian, with his pride, would try to hide it at first but Claude would quickly notice and start to tease him. This would lead to a sort of power struggle between them as they both try to maintain their composure while also trying to get the upper hand in this unexpected tickle situation.
In 'Black Butler Sebastian x Claude fanfic', one of the main themes is the exploration of their complex personalities. They are both mysterious and have their own motives. This is often explored in detail. Another theme is the idea of loyalty. Their loyalty to their masters can create interesting conflicts and situations. And a third common theme is the concept of the supernatural world they inhabit. The rules and hierarchies of the demon world are often part of the stories.
One common theme is family loyalty. Since it's about Sebastian's sister, her loyalty to Sebastian or vice versa is often explored. For example, in many fanfics, she will go to great lengths to protect him or keep his secrets.
One common theme is the power imbalance. Sebastian is a powerful demon, and Ciel is a young, mortal master. This creates a lot of tension and interesting situations where Ciel is constantly trying to hold his own against Sebastian. Another theme is loyalty and its limits. Sebastian is bound by a contract, but his true motives are often questioned.
The theme of sacrifice also shows up a lot. Ciel may sacrifice his own humanity in order to achieve his goals, and Sebastian may sacrifice his own freedom in a way by being bound to Ciel. And there's always the theme of the supernatural, with Sebastian being a demon and all the dark magic and otherworldly elements that come with that in the fanfics.