One common theme is the growth of their relationship. It often starts from a simple acquaintance and develops into a deep emotional bond. For example, they might start as comrades in the fight against demons and gradually fall in love.
The themes usually include unspoken love. Since both characters are not very vocal about their feelings in the original story, fanfictions often explore the idea of them having deep, unspoken emotions for each other. Also, there's the theme of support. They support each other in battles against demons, and this aspect is often elaborated on in fanfictions, like how Shinobu's medical knowledge helps Giyuu when he's injured.
One common theme is the exploration of their different personalities coming together. Tanjiro is very kind - hearted and Shinobu has a more complex, somewhat mysterious nature. Fanfics often show how they bridge that gap.
One common theme is the exploration of their complex relationship. Given Shinobu's vampiric nature and Araragi's human - turned - vampire nature in a way, their relationship is full of power dynamics, loyalty, and a strange form of mutual dependence.
How about 'Giyuu x Shinobu: The Jealousy Factor'? This title clearly shows the relationship between the two characters and the key aspect of jealousy. It gives a hint to the readers about what the fanfic might be about, which is the jealousy that could exist in their relationship.
Adventure can also be a big theme. They might go on quests or face challenges together, which strengthens their relationship. For example, in some fanfics, they have to defeat a common enemy while also dealing with their feelings for each other.
In kira x shinobu fanfic, character development is a crucial element. Both Kira and Shinobu will likely change and grow throughout the story. Their individual personalities, like Kira's perhaps being more impulsive and Shinobu's being more reserved, play a big role in how they interact. Also, the plot often revolves around a central conflict. It could be an external threat like an enemy they have to face together, or an internal struggle within their relationship, such as trust issues.