In a Yu Yu Hakusho and Inuyasha crossover crack fanfic, it could be that the characters are all brought together in a strange dream - like dimension created by a mischievous demon. In this dimension, the rules of both worlds are all jumbled up. Hiei's speed and stealth from Yu Yu Hakusho could be pitted against Sesshomaru's elegance and power in combat. Meanwhile, Kurama's strategic mind has to figure out a way to get everyone out of this dimension. And Kagome is using her spiritual powers to keep everyone's spirits up (literally) while they search for a way to break the spell of the dimension.
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.
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.
One interesting plot could be that Yusuke from Yu Yu Hakusho gets transported to the world of Inuyasha. He teams up with Inuyasha and his group to fight a common enemy, like a powerful demon that is causing chaos in both their worlds. Yusuke's spirit energy could be a great asset in battles against the demons in Inuyasha's world.
One possible plot could be that the female Yusuke has to prove herself in a male - dominated spirit world. She might face discrimination at first but then show everyone her strength and intelligence. For example, she could outwit stronger opponents in battles through her unique strategies.
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.
Yusuke's Rei - gun from Yu Yu Hakusho could be combined with Inuyasha's Tessaiga. For example, Yusuke could use his Rei - gun to enhance the power of Inuyasha's Tessaiga slashes. This would create a more powerful attack against demons. The combination of their raw power would be a force to be reckoned with.
One idea could be an OC who is a half - demon from a previously unknown demon clan. They could have unique powers that both intrigue and worry the main characters. For example, the power to manipulate time on a small scale during battles.
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.
One common theme is power exploration. Since Hiei is a powerful character, fanfictions often explore how his powers interact with new elements in the crossover. For example, in crossovers with other supernatural worlds, how his demon powers compare or combine.
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.
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.