The theme of adaptation is quite common. When 'Blue Exorcist' characters cross over into another world, they have to adapt to new rules and ways of life. For example, in a crossover with 'One Piece', they have to deal with the strange and often wacky world of pirates, new forms of combat, and the concept of the Grand Line. This forces the characters to grow and change in ways that are different from their original story.
A frequently seen theme is the interaction between different character personalities. In a crossover with 'Naruto', for instance, the determined and often hot - headed characters from 'Naruto' meet the more reserved or complex characters from 'Blue Exorcist'. This leads to some interesting character development and relationships. Also, the theme of good vs evil in a new context is common. Since both the original 'Blue Exorcist' and the crossed - over series usually have their own ideas about what is good and what is evil, it creates new scenarios to explore this concept.
One common theme is the clash of different magical systems. Harry Potter has its own kind of magic with wands and spells, while Blue Exorcist has exorcism techniques. Another theme is friendship. Characters from both series often form new friendships or have to deal with differences in how they view friendship. For example, a Hogwarts student might be more used to having friends within their own house, while in Blue Exorcist, friendships are formed based on different criteria at the True Cross Academy.
One common theme is redemption. A character might travel back in time to right their past wrongs. For instance, if a character made a mistake that led to the death of another, they could go back to change that outcome.
One common theme is innocence. Since it's about 'baby Rin', it often shows his pure and naive side before he fully understands the weight of his demon heritage. Another theme is friendship. It might explore how Rin forms his first friendships at the academy.
A frequent theme is the idea of characters from different worlds coming together. For instance, Harry Potter and Rin Okumura might meet and form an alliance. Another theme could be the exploration of good and evil in both universes. In 'Harry Potter', there are dark wizards, and in 'Blue Exorcist', there are demons. Fanfictions might show how the characters from each world deal with the concept of evil. Also, the theme of friendship across different magical realms is quite common, as characters build relationships despite their different backgrounds.
One common theme is the power dynamic between Satan and the other character. Since Satan is a very powerful figure in the Blue Exorcist universe, exploring how that power affects a relationship in a yaoi context can be interesting.
Many Blue Exorcist x Reader fanfictions also explore the theme of redemption. Maybe the reader character has a past they're not proud of, or they encounter a character from the show who is in need of redemption. It can be a really interesting way to develop the story and the relationships between the characters. Also, the theme of loyalty is common. The reader character might have to prove their loyalty to their friends or the exorcist order, and this can lead to some great character - driven storylines.
One common theme is the growth of their relationship. Since Rin is a demon - human hybrid and Shiemi has her own insecurities, their relationship development is often explored. For example, how they learn to trust each other more deeply.
Character development is also a big theme. Writers might take a character from one series and put them in a situation from the other series to see how they grow and change. For example, Danny might find himself in the world of Blue Exorcist and have to learn how to deal with the exorcist's way of fighting demons, which could lead to new skills and a deeper understanding of his own powers.
Love and friendship are also big themes. The relationship between Rin and the OC can range from a slow - burning friendship that turns into love, to an instant attraction. In many stories, they support each other through difficult times, like when facing powerful demons or dealing with the politics within the exorcist community.
In 'Blue Exorcist Rin fanfic', a very frequent theme is Rin's battle against his inner demons, both literally and figuratively. His journey to master his demonic powers while still remaining true to his human side is often explored. Also, his relationship with the exorcist community, whether it's facing discrimination or finding acceptance among them, is a common theme. Additionally, his friendships, especially with the people he meets at True Cross Academy, are often a big part of the stories.