The most popular crossover ideas often involve power combinations. The blue flames of Rin in Blue Exorcist and the spirit gun of Yusuke in Yu Yu Hakusho could be combined in some way in a fanfic. Maybe they create a new super - power to defeat a powerful villain. Also, crossovers might focus on the different mythologies of the two series. The demonology in Blue Exorcist and the spirit realm in Yu Yu Hakusho could be explored together. For instance, how the different types of demons in Blue Exorcist compare to the spirits in Yu Yu Hakusho. There could also be a crossover where the characters are forced to compete in some sort of tournament, which would be really exciting as it would showcase their different abilities.
Well, a really common one is a training - based crossover. Imagine Yusuke training Rin in some of his more unorthodox fighting techniques from Yu Yu Hakusho. Or perhaps the opposite, with Rin teaching Yusuke about demon - slaying weapons. Another popular concept is a friendship - centered crossover. Since both shows have themes of friendship and loyalty, fans might write about the characters becoming fast friends. They could go on adventures together, sharing their different perspectives on the spirit and demon worlds. And of course, there's always the idea of a love - interest crossover, where a character from one show has an unexpected romantic encounter with a character from the other, though this is less common in these types of action - based shows.
First, you need to study both series well. Know the characters' personalities, powers, and backstories. For example, in Blue Exorcist, Rin is a half - demon with a complex relationship with his demon side, and in Yu Yu Hakusho, Yusuke is a delinquent - turned - spirit detective. Then, come up with an interesting plot that combines elements from both worlds. It could be a mystery that requires the skills of both sets of characters. Also, make sure to write good dialogue that reflects the different speaking styles of the characters from each series. If you do these things, you'll be on your way to writing a great fanfic.
A really creative plot is in a fanfic where the afterlife concepts from Yuy Yu Hakusho are merged with the demon hierarchy in Blue Exorcist. The story has Yusuke being sent on a mission by Koenma to help the Blue Exorcist crew deal with a demon that is trying to disrupt the balance between the human world and the demon world in a completely new way. It plays with the idea of different levels of existence and power in both universes.
There's also 'Cross - Realm Defenders'. In this fanfiction, the demon - slaying aspects of both series are the focus. Characters from both shows form a sort of alliance. It shows how Kuwabara from Yuy Yu Hakusho might react to the more formal exorcist system in Blue Exorcist, and how the Blue Exorcist characters would view Kuwabara's unorthodox fighting style. It's full of action and some really cool battle scenes.
One idea could be a 'what if' scenario. For example, what if Yusuke had accepted Raizen's offer to become a full - fledged demon right away. Another idea is to focus on the relationships between the characters in a different way, like a deeper exploration of Kuwabara and Yukina's relationship that was only hinted at in the original. You could also do a time - travel fanfic where a character goes back in time to change a key event in the Yu Yu Hakusho story.
Another plot could involve the Soul Reapers from Bleach and the Spirit Detectives from Yu Yu Hakusho having to exchange their skills and knowledge to deal with a common enemy. For example, the Soul Reapers teach the Spirit Detectives about the use of zanpakuto, while the Spirit Detectives show the Soul Reapers how to better use their spiritual energy in a more physical combat - oriented way. This could lead to some really interesting character interactions and development.
The Miyuki x Yusuke relationship in Yu Yu Hakusho fanfics often delves into the uncharted territories of the story. Fanfic writers might take liberties with their relationship to show how they could grow together. It could be a slow - burn relationship where they start as acquaintances and gradually develop deeper feelings. Or it could be a more instant connection based on a shared goal or a life - changing event that brings them closer.
I think Kurama and Kagome would be an interesting pair. Kurama, with his calm and intelligent demeanor, and his ability to use plants and nature - based powers in combat, could learn a lot from Kagome's knowledge of spiritual powers and the Sacred Jewel. Kagome, on the other hand, could be intrigued by Kurama's complex past as a demon - turned - good. They could exchange ideas about how to deal with demons and use their respective powers more effectively. And their more gentle personalities compared to some of the other characters in their respective shows could lead to some really interesting character - development - focused interactions.
Well, perhaps the Sacred Jewel from Inuyasha somehow gets linked to the Spirit World in Yu Yu Hakusho. Kagome and Inuyasha follow the strange energy trail to the Spirit World. There, they meet Yusuke and Kuwabara. They find out that a powerful spirit in the Yu Yu Hakusho world is corrupted by the dark energy of the Sacred Jewel fragments. So they all have to work together to purify the spirit and restore balance to both worlds. The differences in their fighting styles, like Inuyasha's swordsmanship and Yusuke's martial arts - based spirit attacks, would make for some cool battle scenes.
One popular pairing could be Harry Potter and Botan. It's an unexpected combination that can bring out interesting dynamics. Harry's sense of justice and Botan's role as a guide in the spirit world could create a unique relationship.
Another theme is friendship. Yusuke often forms new friendships or has his ideas about friendship challenged. In crossovers with the Harry Potter world, he might not understand the concept of houses and loyalty at first, but through his adventures, he learns about different types of friendship.