Perhaps in this fanfiction, the 'killer' is targeting specific clans in the Naruto world. This would introduce a whole new layer of conflict, as the targeted clans would have to defend themselves and their legacies. It could also explore how the other characters in the Naruto series react to this threat, whether they choose to side with the killer for some ulterior motives or stand against them to protect their friends and the village.
One interesting idea could be the unique backstory given to the 'killer' character. Maybe they have a dark past related to the Naruto world's hidden secrets.
A plot could involve a group of rogue ninjas with a killer instinct forming an alliance. They target the Hidden Leaf Village with a plan to steal a powerful, ancient artifact. Naruto and his team have to not only stop them but also figure out the true motives behind this alliance. Along the way, they discover secrets about the artifact that could change the balance of power in the ninja world.
One idea could be that the cold killer is a rogue ninja who was once a part of a secret Anbu - like organization in the Naruto world. They have a tragic past that made them turn cold - hearted. For example, they witnessed their entire clan being massacred by a powerful enemy. Another plot could involve the cold killer being hired to assassinate a major Naruto character, but during the mission, they start to question their own motives and actions when they learn more about the target. Maybe they find out that the target is not as bad as they initially thought.
A plot could involve Jeff the Killer being a part of an experiment by Orochimaru. Orochimaru, always interested in unique and powerful beings, tries to combine Jeff's dark powers with some ninja techniques. Naruto and his friends then have to stop Orochimaru's new creation, which is a half - Jeff, half - ninja monstrosity. Along the way, Naruto tries to reach out to the human side of Jeff.
One idea could be that Naruto and Killer Bee have to team up for a special mission that takes them to a mysterious and dangerous land. They have to combine their unique skills, like Naruto's Rasengan and Killer Bee's Eight - Tails powers, to overcome the various challenges they face.
One idea could be a crossover where a powerful Naruto villain somehow ends up in the Fairy Tail world and causes chaos. The Fairy Tail mages would have to use their unique magic to combat the ninja - like abilities of the Naruto character.
Perhaps Naruto is cursed by a powerful jutsu that turns him into an emotionless killer. He begins to hunt down the shinobi who he believes are threats to the Hidden Leaf, not caring about the consequences. However, his friends like Sasuke and Sakura notice the change and try to break the curse. They embark on a journey to find a way to reverse the effect while also protecting Naruto from himself.
One idea could be exploring the backstory of the killer. Maybe they have a tragic past that led them to this paradoxical state of being a killer. For example, they were once a victim of great injustice and now they are taking revenge in a way that defies normal moral and logical boundaries.
One possible plot idea could be the main character having a unique ability that sets them apart in the Naruto universe. For example, they might have a power that can counteract some of the most feared jutsus. Another idea could be a complex relationship with existing Naruto characters, like a rivalry with Sasuke that is based on a different ideology rather than just power.
You could make your OC have a special power related to speed. They could be faster than any of the RWBY characters, which would make for some exciting fight scenes. Their motivation could be to avenge their family who were killed by Grimm. In the fanfiction, they start out as a loner but gradually form bonds with the main RWBY characters as they realize they have a common enemy.
A cool Killer Instinct crossover fanfiction could involve a crossover with a superhero universe. Picture Jago with superpowers like Superman's strength or Spider - Man's agility. Or perhaps a crossover with the Mass Effect universe. The Killer Instinct characters could be part of a galactic battle against the Reapers, using their unique combat skills in a whole new setting.