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.
Kaguya Otsutsuki. She is one of the most powerful characters in the Naruto universe. Her ability to manipulate space and time, along with her other powerful techniques, would pose a huge challenge to Naruto as a martial arts master. Naruto would have to use all of his skills and creativity to even stand a chance against her.
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.
Madara Uchiha would be a formidable foe. His Sharingan abilities are second to none and his experience in battle is vast. He could easily outwit and overpower the team with his powerful jutsu and strategic mind. Plus, his connection to the Uchiha clan's history gives him an edge in understanding and countering the team's moves.
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.
His toughest opponent could be Gary. Gary has always been a rival to Ash and has a strong and diverse team of Pokemon. Their battle would be intense as they both know each other's battling styles well.
Sasuke could still be a tough opponent. Even though Naruto has all elemental affinities, Sasuke has his own powerful Sharingan and Rinnegan abilities. He can manipulate Amaterasu and other powerful techniques. Their rivalry would be intense as Naruto tries to use his newfound elemental powers against Sasuke's ocular powers.
Sasuke could still be a tough opponent. Even though Naruto can use all elements, Sasuke has his own powerful abilities, like his Sharingan and Rinnegan. He's very skilled in ninjutsu and genjutsu. Another tough opponent might be Madara. He has vast experience and extremely powerful jutsu. Naruto would have an advantage with all elements, but Madara's knowledge and power would make him a formidable foe.
Jiren could still be a tough opponent in the 'vegito tournament of power fanfiction'. His power level is extremely high, and he has great combat skills. Even for Vegito, defeating Jiren would be no easy feat as Jiren has proven his strength time and again in the original tournament.
Loki would also be a difficult adversary. He's very cunning and could use his tricks and illusions to try and outwit Goku. Goku would need to rely on his senses and ki to see through Loki's deceptions.
Another option could be a mortal martial arts master who unknowingly stumbles upon Percy. The mortal is a master of a rare and ancient form of martial arts. At first, he is hesitant to teach Percy because he senses something different about him. But as he gets to know Percy better, he realizes Percy's good heart and decides to train him. This would add an interesting dynamic as Percy has to balance his demigod life with his training under a mortal.
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.