It could be an enemy who has the power of sound. This opponent might be able to create sonic blasts with their music. Naruto would have to figure out how to counter these sound - based attacks, perhaps by using his wind - style jutsu to disrupt the sound waves.
Another possibility is an evil music - loving sorcerer. This sorcerer uses enchanted instruments to cast spells. Naruto would have to be careful not to get caught in the magical melodies. He could use his shadow clones to distract the sorcerer while he figures out a way to destroy the enchanted instruments and end the fight.
Sasuke would likely be a very tough opponent. He has his Sharingan and is extremely skilled in ninjutsu. His rivalry with Naruto also means he would be highly motivated in a fight.
Madara Uchiha. He is extremely powerful with his Sharingan and Rinnegan abilities. Even if Naruto is a martial arts master, Madara's vast array of jutsu and his strategic mind would make him a formidable foe. He could use his space - time ninjutsu to avoid Naruto's attacks and then counter with powerful attacks of his own.
It could be an unknown brother that was kept secret all this time. Maybe he was hidden away by some enemy of the village and trained to be a powerful adversary for Naruto.
His friends like Sasuke could be his allies. Sasuke, despite his own complex journey, has a deep connection with Naruto. He would understand Naruto's need to fight against the council if they were being unjust. Also, Sakura, with her medical skills and growing strength as a ninja, would be a great asset. She could heal the team during any confrontations and also use her combat skills when necessary.
One possible plot is that Naruto's brother is jealous of Naruto's status as a hero. He decides to challenge Naruto to prove himself. The fight might start with some basic exchanges of punches and kicks. But as it progresses, they both start using their more advanced ninja skills. Naruto may struggle because he doesn't want to hurt his brother too much, but he also knows he has to stop his brother's dangerous actions.
One possible plot could be that Naruto is mislead into believing his family has been possessed by some dark force. So, he has to fight them to free them. For example, an evil ninja might use a genjutsu on his family members, making them act against Naruto's will.
In a Naruto - music fight fanfiction, the music could have magical properties. Maybe an enemy uses a song to put Naruto in a trance, but Naruto then turns the tables by using his own chakra - infused song. He could call upon the power of the Nine - Tails and combine it with the energy of the music to create a super - powerful attack.
Maybe an ancient evil ninja who was sealed away long ago but has now been released. This antagonist could have powers that nullify any communication or summoning jutsu, forcing Naruto to fight alone. He would be extremely powerful and know many forgotten and deadly jutsu.
I think Gaara could be a really tough opponent for Naruto in an MMA - like scenario. Gaara has great control over his sand, which could be used defensively and offensively in a fight. He's also very resilient and has a lot of experience in combat. His stoic nature would make it difficult for Naruto to read his moves and find an opening. And Gaara's determination to win is on par with Naruto's.
It could be Sasuke. Sasuke is highly intelligent and has a great strategic mind. He would approach shogi with the same intensity as he does his ninja battles. He would be able to analyze Naruto's moves quickly and counter them effectively.