In the fanfiction, a plot twist could be that Naruto's smithing skills attract the attention of an enemy. This enemy wants to use Naruto's skills for their own evil purposes. But Naruto, being Naruto, refuses and uses his newly acquired smithing knowledge to create weapons and defenses specifically designed to counter this new threat.
One possible plot twist could be that Naruto discovers he has a natural affinity for a special type of metal that is only found in a hidden realm. This metal has unique properties that can enhance his ninja tools in unexpected ways. Another could be that the smithing skills he learns are actually part of an ancient prophecy. It was foretold that a ninja with Naruto's spirit would master smithing to create a weapon that could save the ninja world from a great threat that is yet to come.