One possible plot is that Harry, as a blacksmith, is sought after by all the magical communities. He could create a new kind of portkey that is not only more stable but also has the ability to transport large groups of people at once. His blacksmithing skills could also be used to create a device that can detect dark magic from a distance. This would make him a valuable asset in the wizarding world. He could also hold blacksmithing workshops at Hogwarts, teaching the younger students the art of creating magical items, which would be a great way to pass on his skills and knowledge.
One possible plot could be that the Naruto world is facing a new threat that can only be defeated by unique weapons. Harry Potter, the blacksmith, arrives in the Naruto world. He starts to study the materials and techniques available in this new world. He combines his knowledge of magic - enhanced blacksmithing with the ninja's understanding of chakra. As he creates these new weapons, he also forms bonds with the Naruto characters. They train together to master the use of these new weapons, and ultimately they face the great threat together, with Harry's blacksmith - made weapons playing a crucial role in the victory.
It could be an old wizard who has been living in seclusion for centuries, perfecting his smithing skills. He might emerge during a time of great need in the wizarding world, like when Voldemort was at his most powerful.
Perhaps Hagrid could be his mentor. Hagrid has a lot of knowledge about magical creatures and their parts, which could be useful for Harry in his blacksmithing. He could show Harry how to use things like dragon scales or unicorn hairs in his creations.
Perhaps a godlike blacksmith in Harry Potter fanfiction has the ability to infuse emotions into their creations. So a piece of armor they make might give the wearer courage or a sense of calm. This would add a new dimension to the magical items in the Potterverse, as they usually just have more straightforward magical functions.
I'm not entirely sure if there is a specific one with that exact premise. However, fanfiction authors are constantly coming up with new and unique concepts. Given the popularity of both Harry Potter and Naruto, it's possible that someone has written a story where Harry Potter is a blacksmith within the context of a Naruto fanfiction. It could be an interesting take on the characters and their worlds, perhaps exploring how Harry's skills as a blacksmith could interact with the ninja world of Naruto.
One interesting storyline could be a blacksmith who discovers a magical metal only found in the Forbidden Forest. He uses it to create a powerful wand that has unique abilities. Another could be a blacksmith who is actually a descendant of an ancient wizarding family of blacksmiths and is on a quest to recreate the lost magical artifacts of his family.
One idea could be that Mewtwo somehow gets transported to the Harry Potter universe. It could be mistaken for a very powerful magical creature. Harry and his friends could initially be wary of it, but then realize it has no intention of harm and they team up to fight against the dark forces in the wizarding world using Mewtwo's psychic powers.
Another plot could be that Voldemort discovers Powergirl's existence and sees her as a threat. He tries to manipulate the wizarding world against her. But Harry and his friends believe in her and together they work to expose Voldemort's lies and also learn from Powergirl about different ways of fighting evil. Powergirl could introduce new concepts of justice and heroism to the wizarding world.
One possible plot could be that Harry discovers a hidden magical realm in the Pokémon world during one of his adventures. There, he meets Rayquaza. Rayquaza, being a powerful Pokémon, senses the strong magic within Harry. Together, they have to fight against a dark wizard who has found a way to corrupt Pokémon and use them for evil purposes in both the wizarding world and the Pokémon world.
Maybe the Timekeeper is actually a descendant of an ancient wizarding family who was tasked with keeping time in balance. In the fanfiction, Voldemort discovers this and tries to use the Timekeeper's powers to go back in time and prevent his own downfall. The Timekeeper has to fight against Voldemort's minions while also trying to find a way to protect the timelines. Harry and his friends get involved when they notice strange time - related disturbances around the school. They join forces with the Timekeeper to stop Voldemort's evil plan.