It might be an old enemy of Naruto who has come back for revenge. For example, someone from the Akatsuki who wasn't completely defeated before. They could target Naruto again and put him in a life - threatening situation, which makes Kurama cry.
It could be related to Naruto's struggle with his own identity. Naruto has always been an outcast in the village to some extent. If he starts to doubt himself or his place in the world, Kurama, being a part of Naruto, might cry seeing Naruto's internal turmoil.
The antagonists could be a secret organization within the Hidden Leaf Village itself. They are afraid that if Kurama is freed, it will bring destruction to the village. They think Naruto is being reckless and they will do anything to stop him, including framing him for crimes he didn't commit and turning his friends against him, all in an effort to keep Kurama sealed.
The Kurama Brothers could be entities that have a similar origin to Kurama. They might have been part of the same chakra - based creation in the Naruto universe. They could possess some of Kurama's traits like great power and a connection to the natural energy in the world. They are often depicted as having their own personalities, with one being more hot - headed and the other more calm and calculating. Their relationship with Naruto usually involves them using their powers and knowledge to help Naruto grow as a ninja and a person. They might also have their own goals and motives that are intertwined with Naruto's journey.
It might be Iruka - sensei. Iruka was one of the first people to recognize Naruto for who he was and give him a sense of belonging. If Iruka were to die, Naruto would remember all those early days of support and kindness. His death would break Naruto's heart as it would be like losing a part of his family, the family that accepted him when no one else did.
It could be an ancient and powerful god or goddess who has a grudge against Percy. Maybe they see Percy as a threat to their power or plans, so they plot his death. For example, a forgotten deity who wants to regain their former glory by getting rid of Percy, who is often seen as a hero in the Percy Jackson universe.
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.