There could be a group of ninja from a hidden village that has always been envious of Konoha's power. When they hear about Naruto being blessed by Amaterasu, they see it as an opportunity to weaken Konoha. So, they send their best assassins to either kill Naruto or steal his power, making this group the main antagonists in the fanfiction.
Maybe a powerful rogue ninja who has discovered a way to steal the powers of divine entities. He sets his sights on Amaterasu and Naruto because he knows that if he can combine their powers, he will be unstoppable. He creates various traps and illusions to separate them and weaken them so that he can carry out his evil plan.
It could be a rogue ninja who has also discovered a way to harness the power of Amaterasu in a corrupt way. This ninja might want to use the power to take over the ninja villages and Naruto stands in his way.
Perhaps a powerful demon that has a grudge against the power of Amaterasu. It targets Naruto because he is the one who seems to be getting closer to Amaterasu. The demon could be from an ancient time when Amaterasu was first used to seal it away, and now it's back to stop Naruto from fully understanding and using Amaterasu.
The fanfiction might revolve around Naruto being blessed by Amaterasu during a crucial battle. This sudden blessing turns the tide of the fight. His friends and allies are initially shocked but then realize that this could be a sign of something greater. Naruto then has to learn to control this power quickly, and he might face some internal struggles as the power has its own will. He may also have to face those who are jealous or afraid of his new power.
Maybe it's an organization that believes the dragon's blessing is an abomination to the wizarding world's natural order. They see Harry as a threat and will stop at nothing to eliminate him and the power he has received. They could be a secret group within the Ministry of Magic that has its own radical ideas about magic and purity.
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.