A new character could also be the main antagonist. For example, a mysterious figure who has been observing the Naruto world from the shadows for years. This character could have mastered all types of chakra natures and created a new form of jutsu that allows him to control large amounts of people at once. He starts his takeover by causing chaos in the major villages and then moves in to establish his own rule.
Madara Uchiha is another obvious candidate. Even though he was defeated in the original story, in a fanfic he could be resurrected with even greater power. His Sharingan abilities and his knowledge of the ninja world's secrets would make him a formidable antagonist for taking over the Naruto world. He could manipulate different clans and villages against each other before making his move to take over.
Well, perhaps a group of outcast ninjas who have been mistreated by the major villages join forces. They develop a new and powerful fighting style that no one has seen before. They start by infiltrating the smaller villages, taking them over quietly, and then use the resources and manpower of those villages to launch a full - scale attack on the larger ones, aiming to take over the entire Naruto world.
A powerful rogue ninja who has discovered a way to control digital creatures could be the main antagonist. This ninja might see Impmon as a key to unlocking even greater power and would stop at nothing to capture him. He could use his knowledge of ninjutsu and his new - found ability to manipulate digital entities to create chaos in both the ninja world and potentially the digital world if he were to gain access to it through Impmon.
It could be a rogue ninja who has mastered parkour to an extreme level. He uses his skills to steal important ninja artifacts and Naruto has to stop him. This antagonist would be very fast and agile, making it difficult for Naruto to catch him using normal ninja techniques.
Orochimaru could be a great antagonist. He might want to use Shenron's power for his own dark experiments. He could try to steal the dragon balls and manipulate the wishes to gain immortality or create an army of powerful, immortal ninja.
It could be someone from Miku's world who follows her into the Naruto universe and has evil intentions. Maybe a jealous rival who wants to stop Miku from becoming popular in the new world.
It could be a rogue ninja who wants to use the mummy's power for evil. He might have discovered some ancient scrolls that told him about the mummy's potential and is now trying to awaken it fully and control it to take over the ninja world.
It could be Orochimaru. He has always been interested in the power of the dead and bringing back the strong. He might have found a way to reanimate Naruto for his own evil purposes, like using Naruto's power to take over the ninja world.
A mysterious figure within Konoha who has a hidden agenda. This person doesn't like the idea of Naruto breaking the norms and wearing a dress. They secretly manipulate events to make Naruto's life difficult while he's in the dress. For example, they might arrange for Naruto to be in situations where his dress - wearing causes maximum embarrassment, all in an attempt to make him conform to the traditional ways.
A powerful and corrupt ninja from another village could be the main antagonist. He hears about the unique knives in Naruto's village and decides to steal them to increase his own power and the dominance of his village. He would stop at nothing to get his hands on these knives, even if it means starting a war between the villages.
It could be a new character. Maybe someone who has been influenced by the dark side of the ninja arts and wants to overthrow the current order. This new antagonist might be jealous of Naruto's success and the peace in the village, and they strive to create chaos.