It could be a rogue ninja who also has a connection to the Raijin Sword. Maybe he was once a guardian of the sword but was corrupted by its power and now wants to use it to rule the ninja world.
It could be Orochimaru. He's always been after powerful ninja tools and the Shusui Sword would be no exception. He might want to use it to enhance his own jutsu or gain more power in the ninja world.
In some Naruto Raijin fanfiction, Kakashi could also be an important character. He might play the role of Naruto's mentor in understanding Raijin's powers or techniques. For example, he could help Naruto analyze Raijin's lightning - based attacks and come up with counter - strategies.
The Raijin Sword could also have some sort of connection to Naruto's chakra or his Nine - Tails power. It could potentially unlock new forms or power - ups for him. For example, the sword's energy could combine with the Nine - Tails' chakra, creating a unique and extremely powerful attack.
A powerful demon could be the main antagonist. This demon senses the power of the Yoru sword and wants to possess it. It starts causing chaos in the ninja world, and Naruto has to face this demon with the help of the sword. The demon could have some special abilities that make it difficult for Naruto to fight, like the ability to manipulate shadows or create illusions that are even stronger than normal ninja illusions.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.
It could be a rogue ninja who also has somehow acquired bankai - like powers. This antagonist might be jealous of Naruto's newfound abilities and wants to prove that he is the stronger one. So he starts causing chaos in the ninja world, challenging Naruto at every turn.
It could be a rogue ninja who wants to disrupt Naruto's business to gain power. Maybe he's jealous of Naruto's success and popularity in the business world. He could try to steal Naruto's business secrets or sabotage his operations.
It could be a rogue Kumo ninja who is jealous of Naruto's power and wants to prove himself by defeating Naruto. This ninja might have some dark secrets and a grudge against Kumo as well.