It could be another summoner who is jealous of the panther summoner's power. This antagonist summons a powerful and dangerous beast, like a giant snake, to constantly challenge the panther summoner and try to steal their power or reputation.
A group of rogue ninjas who want to capture the panther and sell it on the black market for a huge sum of money. They are constantly hunting the panther summoner, setting traps and ambushes. They don't care about the consequences for the summoner or the balance of the ninja world, they are just after the profit from the rare panther.
The main antagonist could be one of the village elders. They might see Naruto's fox - summoning abilities as too dangerous and banish him out of fear. Throughout the fanfiction, they could try to prevent Naruto from coming back to the village, fearing that he'll seek revenge or disrupt the power balance they've established.
An old rival of T'Challa's from his training days could be the main antagonist. This person was always jealous of T'Challa's status as heir to the throne and the future Black Panther. Now, he has found a way to enhance his own abilities and wants to prove that he is the better warrior by taking down T'Challa and claiming the Black Panther mantle for himself. He would know T'Challa's fighting style well, making him a very challenging opponent.
It could be someone who wants to steal the knowledge Naruto gained from the Black Panther. Maybe a rogue ninja or a power - hungry villain from another land.
A group of Wakandan nobles who oppose Shuri's rule as Black Panther could be the main antagonists. They believe that the Black Panther should be a more traditional figure and that Shuri's focus on innovation and change is a threat to their long - held power and status. They might plot to undermine her at every turn, from spreading false rumors within Wakanda to allying with external forces against her.
It could be a rogue Wakandan scientist who is jealous of the daughter's unique abilities. He might see her as a threat to his own plans of taking over Wakanda's technology and using it for his own evil purposes. So, he tries to undermine her at every turn and stop her from fulfilling her destiny in Wakanda.
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 an ancient evil spirit that was once bound but is now trying to break free by using the summoner as a conduit. This spirit could be manipulating events from the shadows, causing chaos in the wizarding world to draw the summoner out.
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.