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.
Another possibility is Kakashi. Kakashi is experienced and very smart. He has seen all kinds of battle strategies and could apply that knowledge to shogi. He might also be a bit of a trickster in the game, using his Sharingan - like observational skills (even if not the actual Sharingan in a shogi context) to read Naruto's every move and plan his own accordingly.
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.
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.
One possible plot could be that Naruto, being new to shogi, struggles at first. But then, using his determination from his ninja training, he starts to study the game intently. He might meet an old shogi master in the Hidden Leaf Village who teaches him unique strategies. As he improves, he challenges his friends to shogi matches, and through this, it shows his growth not just in the game but also in his ability to strategize in general.
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.
Sasuke could be a tough opponent. He has his own natural athleticism and competitive spirit. He would be very strategic on the football field, always looking for ways to outwit Naruto.
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.
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.
It could be a new character created specifically for the fanfiction. A trainer with some super - rare and powerful Pokémon that Ash has never faced before. This trainer might have unique battle tactics that really test Ash's skills.
It might be another new and mysterious trainer in the Kalos league. This trainer could have some rare and never - before - seen Pokémon with unique abilities. But Ash, through his experience and the growth of his Pokémon over time, is able to find the weaknesses of these new Pokémon and defeat this tough opponent. His ability to adapt to different battle situations and his Pokémon's loyalty are what make him victorious.
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.