Another option could be a jealous rival at the Ninja Academy. This rival is envious of Naruto's natural talent and charisma, even though Naruto is an outcast. He tries to undermine Naruto at every opportunity, from spreading false rumors to trying to get him in trouble with the teachers.
It could be a bully in the village. This bully could constantly pick on Naruto because of his status as the Jinchuriki of the Nine - Tails. He might try to steal Naruto's food or prevent him from playing with other kids.
It could be a relative of one of Naruto's old foes. For example, a nephew of Orochimaru who has been taught by Orochimaru's remaining followers. He might be jealous of the peace Naruto has brought and wants to disrupt it by targeting his kid.
The Reverse - Flash would be a great antagonist. His time - traveling abilities and his vendetta against the Flash family could make for a really intense story, especially if he teams up with other villains from the Kid Flash's universe in a crossover.
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.
Perhaps a long - lost relative of Voldemort who has been in hiding all these years. This relative has discovered a way to gain power through Harry's kid. They might start by infiltrating Hogwarts under a false identity and slowly manipulate events to get closer to the kid. Their ultimate goal could be to sacrifice the kid in a dark ritual to bring back the dark days of Voldemort's reign.
The Green Goblin could be a great antagonist even when Peter is just a kid. He could be starting out his evil deeds in a more low - key way that still catches Peter's attention. Maybe he's stealing from small businesses in Peter's neighborhood, and Peter starts to suspect something is up.
It could also be a new villain. Maybe someone who has been waiting for Sonic to be in a vulnerable state. This new villain could have a special power or weapon that is more effective against Sonic as a kid. For example, a device that can slow down Sonic even more or nullify his powers temporarily. They might target Sonic not just because of his weakened state, but also because they know that his friends will be distracted trying to help him get back to normal.
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.