Well, it could start with a common enemy. There could be an entity that is a threat to both Inuyasha's world and Riddick's. This would force them to team up. For example, a powerful demon in Inuyasha's world has some sort of connection to a dangerous alien race in Riddick's universe. They would have to combine their combat skills, with Inuyasha's swordsmanship and Riddick's hand - to - hand combat abilities.
Inuyasha is impulsive and hot - headed. Riddick is more of a lone wolf, brooding and calculating. Their initial meetings would probably be full of tension. Inuyasha might see Riddick as a threat to his territory or his friends. Riddick would likely be suspicious of Inuyasha's demon side.
Survival is a common theme. Since Riddick is often in harsh environments in his own story, when crossed over into other universes, he still has to fight to stay alive. For example, if crossed with 'The Hunger Games' universe, he'd have to use his skills to outwit the other tributes and survive the deadly games.
One idea could be that Sephiroth somehow gets transported to the feudal era of Inuyasha. He might initially be seen as a great threat due to his power. But as the story progresses, he could form an unlikely alliance with Inuyasha and his group against a common enemy, like a powerful demon that is even more menacing than any they've faced before.
One possible plot could be that Riddick somehow gets transported to the Naruto universe. He could initially be seen as a threat, but then he teams up with Naruto and the others to fight a common enemy, like a new powerful Akatsuki member who has powers similar to some of the creatures in Riddick's world. Riddick's combat skills and his ability to see in the dark could give the Naruto gang an edge in battles.
One great 'riddick crossover fanfiction' could be where Riddick crosses over with the 'Alien' universe. In such a story, Riddick's survival skills would be put to the ultimate test against the deadly xenomorphs. His ability to see in the dark would give him an edge, but the aliens' strength and stealth would still pose a huge threat.
A popular one is 'The Meeting of Paths'. This fanfiction focuses on a situation where Inuyasha and Sesshomaru meet under different circumstances than in the original series. It has great characterizations, and the way the two interact and gradually change their views of each other is really well - written.
One idea could be that Sakura, with her medical ninjutsu skills, ends up in the Feudal era of Inuyasha. She could help treat the wounds of the villagers who are often attacked by demons. Inuyasha might be initially suspicious of her strange powers but gradually come to respect her as she proves her worth in battles and in healing the injured. For example, when Inuyasha gets seriously injured in a fight against a powerful demon, Sakura's medical skills could be the key to saving him, which would change his view of her.
Power and growth. Both Inuyasha and Bleach have characters with different levels of power. In crossovers, often a character from one series has to adapt and grow in the other's world, facing new challenges that test their power.
Characterization can be tough. Each character from both series has their own distinct personality. The Avengers have their own dynamics, and Inuyasha has his own temperamental nature. When bringing them together, the writer has to ensure that the characters stay true to themselves while also developing new relationships. For instance, Captain America's sense of justice might clash with Inuyasha's more self - centered approach to fighting at first.