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.
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.
One idea could be that the panther summoner is from a hidden clan in Naruto's world. They are initially shunned because of their unique summoning ability. But as the story progresses, they prove their worth in battles against powerful enemies, like during the Akatsuki attacks. The panther could have special abilities like being able to disappear in the shadows and surprise opponents.
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.
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 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.
Often, the main character would be the phoenix summoner themselves. They are likely to be a young ninja with a special talent for summoning phoenixes. Maybe there's also a mentor figure, an older ninja who guides the summoner in understanding their powers.
Obviously, Naruto would be one of the main characters. Since it's a fanfiction about him as a dragon summoner. There might also be his friends like Sasuke and Sakura. Sasuke could be intrigued by Naruto's new power and might have a complex relationship with it, either being competitive or even learning from Naruto in some way. Sakura could be worried about the risks Naruto takes with this new and dangerous power.
Well, Naruto is likely to be one of the main characters, as it's in the title. He would be the one learning to be a griffin summoner. There might also be the griffins themselves, which could be characters in their own right. They may have different personalities and abilities. Another possible main character could be a mentor figure who teaches Naruto about griffin summoning.
It could be a villain who is after Beast Boy's powers. Since Beast Boy is getting trained by Black Panther and becoming stronger, this villain might see him as a threat. Maybe it's someone like Deathstroke who wants to stop Beast Boy from reaching his full potential under Black Panther's tutelage.