Perhaps it's a group of mercenary ninjas. They've been hired by someone who has a grudge against Zabuza. They see Zabuza as an easy target when he's alone or vulnerable, but Naruto shows up to spoil their plan.
There might be a rogue ninja who was once defeated by Zabuza and now wants revenge. This ninja has been training hard and thinks he can finally get his revenge. But Naruto, with his determination to save Zabuza, stands in his way. Naruto's growth as a ninja and his never - give - up attitude would be put to the test against this antagonist.
Perhaps a new and original antagonist could be created for this fanfiction. This antagonist could be a rogue ninja who has mastered a forbidden jutsu that allows him to manipulate time as well. He could be the one who sets up the events that lead to Minato's endangerment, and Naruto has to figure out his motives and stop him while also saving Minato.
It could be a rogue ninja who is using the poochyena for some sort of evil experiment. Naruto has to stop this ninja from harming the poochyena.
It could be a rogue ninja from Tobirama's time who has a vendetta against the Senju clan. This ninja might have been waiting for the perfect moment to strike at Tobirama when he is alone or vulnerable. Naruto then has to face this powerful and vengeful enemy to save Tobirama.
In this fanfiction, Naruto might use his unique jutsus like the Shadow Clone Jutsu. He could arrive just in time to stop an enemy from killing Zabuza. Maybe Naruto realizes Zabuza's potential for redemption and decides to protect him.
Maybe an old enemy of Madara who realizes that by targeting Naruto, he can get back at Madara. Since Madara has some sort of connection or interest in saving Naruto in this fanfiction, the enemy thinks that if he can harm Naruto, he can either draw Madara out or simply cause him pain. This enemy could be a powerful rogue ninja with a vendetta against Madara.
It could be a new, self - made villain. Maybe a rogue ninja who was once rejected by the Hidden Leaf Village. He blames Naruto for his misfortunes and decides to target his sister as revenge. This villain would have his own set of unique and dangerous jutsu, which Naruto would have to figure out how to counter in order to save his sister.
It could be someone like Madara Uchiha. He's extremely powerful and his goals are often in conflict with Naruto's. Goku would have to face his Sharingan abilities and his vast array of jutsu.
It could be Palpatine. Since Anakin is going against his wishes by saving the younglings, Palpatine would see this as a threat to his plans. He might send more clone troopers or even some of his dark side minions to stop Anakin.
Maybe Kabuto. He's a sly character who often works for the more powerful villains. He could be sent to capture Naruto's younger self for some experiment or to gain information. Naruto would need to fight against Kabuto's medical ninjutsu and his sneaky tactics to save his younger self.
It could be a rogue ninja from the Hidden Leaf's past who has come back for revenge. Maybe someone who was wronged by the village long ago and has now amassed great power to destroy it. Another possibility is a new enemy from outside the village, like a powerful warlord from a far - off land who wants to conquer the Hidden Leaf for its resources.