Who could be the main antagonist in 'naruto is aku fanfiction' if Aku is not the main antagonist?

Aku could be a storyteller. Maybe it's a person's name.

One possible plot could be that Aku somehow ends up in the Naruto universe. Naruto, being a fan, is initially excited but then realizes that Aku's presence is causing chaos. So, Naruto has to find a way to either send Aku back or find a way to co - exist with Aku without letting Aku's evil influence overtake the ninja villages. He might try to reform Aku using his own positive attitude and the power of friendship, which is a common theme in Naruto.

One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.

A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.

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.

Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.

Kabuto could also serve as the main antagonist. He is a sly character. He might enter the race with the intention of gathering data on all the participants, especially Naruto and his friends. He could use his medical ninjutsu to enhance his own racing abilities in unfair ways, like numbing pain to push himself further or using some sort of drug - like jutsu to boost his speed.

A powerful rogue ninja who wants to steal the Nanosuit from Naruto could be the main antagonist. This ninja might have his own reasons, like using the suit to take over the ninja world.

It could be one of the powerful Deadman characters who see Naruto as an intruder and a threat to their world.

It could be a new villain who has emerged during Naruto's absence. This person might be after the power that Naruto holds or wants to destroy the Hidden Leaf Village for some unknown reason.