Well, one possibility is that Naruto is turned into a robot by some kind of advanced jutsu gone wrong. He might then struggle to adapt to his new mechanical form. For example, he could find it hard to perform his regular ninja moves in the same way. Maybe he would initially be rejected by his friends until they realize his inner self is still the same Naruto they know and love.
It could be Kabuto. Since he has a lot of knowledge about medical ninjutsu and experiments, he might see Naruto's robotic form as a threat or something he can use for his own evil purposes. He could try to control Naruto's robotic functions and turn him against his friends.
At first, Naruto might be a bit cautious. He's used to facing opponents in battle, and a strange new object like a robot could potentially be dangerous. But once he realizes it's not an immediate threat, he'd likely approach it with his usual enthusiasm. He might start asking it all sorts of questions, like 'What are you?' and 'Where are you from?' His reaction would be a mix of excitement and wariness.
Well, one possibility could be that Tails is forced to become a robot against his will. Maybe an evil scientist captures him and turns him into a robotic servant. He could then struggle to regain his free will and his original form. He might use his mechanical skills, even in his robotic state, to outwit the scientist and find a way to reverse the process.
One possibility could be that Naruto is drawn to boxing because of its physicality and the determination it requires. He might see it as a new way to test his strength and endurance outside of ninja battles. Maybe he starts training with a local boxing coach who sees his potential and helps him develop his skills.
Perhaps it's for exploration. Naruto is always curious about the world outside. The robots can be sent to explore dangerous or uncharted areas where it might be too risky for humans to go. They can collect information and report back to Naruto.
Maybe Naruto has some knowledge from an unexpected source. For example, he could have met a wandering inventor who taught him the basics of robotics. With his determination and quick learning ability, he then starts building the robot army. He might also use his chakra in some unique way to power or control the robots.
If Tails becomes a robot in a fanfiction, his personality might become more logical and less emotional. Since robots are often programmed to be rational, he might start to analyze situations more coldly. For example, he might not be as quick to jump into a rescue mission out of pure compassion like he used to.
One possibility is that Naruto, with his determination, could adapt Zeref's powers in a unique way. Maybe he'd use Zeref - like magic to enhance his ninjutsu. For example, he could infuse Zeref's death - based magic into his Rasengan, creating a more powerful and dangerous attack. Another aspect could be Naruto's personality change. If he becomes influenced by Zeref, he might become more brooding and less of the cheerful ninja we know. However, his core values of friendship and protecting the village would still likely be there, just warped a bit by Zeref's dark nature.
One possibility could be that Naruto is transported to the Mortal Kombat universe and through some magical or technological means, he takes on the powers and identity of Sub - Zero. Maybe he has to adapt to the new combat styles and the brutal nature of the Mortal Kombat world while still retaining some of his Naruto - like traits such as his determination and never - give - up attitude.