He could be a long - lost relative of one of the characters. For instance, if he's related to Bill, it could create a whole new dynamic. Rav might try to reconcile Bill and the Bride, or he could be caught in the middle of their feud, not really knowing which side to take.
Well, it could be about Rav getting involved in the revenge plot of Kill Bill. Maybe Rav has some unique skills that help the Bride in her quest for vengeance. For example, Rav could be a master of some ancient martial art not seen in the original Kill Bill universe and uses it to fight off the Crazy 88s or other enemies.
He could fit in as a special guest lecturer. His knowledge of different magics would make his classes very popular. Students would be eager to learn from him, especially about his adventures in Middle - earth.
Deadpool would likely be a very chaotic element in the Naruto world. In a fanfic, he might arrive through a portal that was opened accidentally. His combat style would be completely different from what the ninjas are used to. He doesn't really have a set of jutsu like the ninjas, but his acrobatic fighting and use of guns and swords would be a novelty. He could form an unlikely friendship with Naruto. Naruto would be intrigued by Deadpool's immortality and try to understand it, while Deadpool would be interested in Naruto's chakra and the whole concept of ninjutsu.
He could be a new type of hero. Maybe he starts as an unknown entity and quickly rises in power and reputation. His unique jutsu like the Sharingan (if he somehow acquired it in the crossover) would make him stand out. He could join forces with the Avengers and teach them new combat techniques based on his ninja skills.
He could be a mysterious outsider. Just suddenly appear in the Naruto world and start causing trouble. Maybe he's looking for something in the Naruto universe, like a powerful chakra source or a special jutsu. And the ninja have to figure out who he is and what he wants before he destroys everything.
CC could potentially be quite powerful. If we consider her abilities from her original context, she might be on a similar level to some of the stronger jonin in the Naruto world. For example, her ability to analyze and adapt quickly could give her an edge in combat.
In a fanfic, Fem Naruto could be a part of a secret organization in the Highschool DxD world. This organization could be focused on maintaining the balance between the different supernatural forces. She would bring her ninja training and knowledge, which includes things like espionage, combat tactics, and survival skills. Her presence would disrupt the status quo and lead to new alliances and rivalries as different characters react to her unique abilities and approach to problems.
Well, Juvia's water magic could be quite interesting in the Naruto world. Her ability to control water could be seen as a unique form of ninjutsu. Maybe she could team up with some water - style ninjas and learn from each other. For example, she could show them her more creative uses of water like creating large water animals, while they could teach her about chakra control in water - based techniques.
He could fit in as a new and mysterious student. With his godlike powers, he would quickly become a figure of interest. Maybe he'd be scouted by different factions in the DxD world, like the devils or the fallen angels, who see his potential as an ally or a threat.
A Saiyan could fit in as a mysterious and powerful outsider. They would likely be feared by some and revered by others. Their Saiyan abilities would set them apart from the normal humans in Westeros.