Maybe it's a group of shinobi from a rival village. They see Naruto as a threat to their village's power and influence. With him in the Bingo Book, they think it's a legitimate chance to eliminate him and send a message to Konoha.
Maybe it's a powerful enemy from outside the village who has infiltrated the bingo book system. They see Naruto as a major threat to their plans and so secretly added him to the book, so they can send their minions to take him out without raising too much suspicion.
The plot might involve Naruto going undercover while being in the Bingo Book. He could be trying to find out who put a high price on his head and why. Along the way, he may encounter old friends who are conflicted about his situation in the Bingo Book.
Well, Naruto is surely one of the main characters as the title implies it's centered around him. His friends like Sasuke and Sakura are likely to also be important characters, as they are usually involved in Naruto's adventures.
A new character could be created as the antagonist. For example, an Uchiha who has been corrupted by the power of the Sharingan and wants to use Naruto's special Uchiha - related abilities for his own evil purposes. This character could be jealous of Naruto's unique position within the Uchiha - related story and try to take it from him.
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.
Madara could still be the main antagonist even in a 'naruto bankai fanfic'. He senses Naruto's new power and realizes it could be a threat to his plans of world domination. He adapts his strategies and attacks Naruto with all his might, trying to prevent Naruto from fully mastering and using his Bankai - inspired power to its fullest potential.
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.
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.
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 group of ninja who oppose the new order of the Naruto Empire could be the main antagonists. They might believe that the empire has become too controlling or has strayed from the original values of the ninja world. They could use guerrilla tactics and create internal unrest within the empire to try and bring it down.