A Naruto blacksmith fanfiction could also focus on a blacksmith who has the power to infuse their own life force into the weapons they create. This makes the weapons extremely powerful but also comes with a great risk to the blacksmith's health. The story could revolve around the blacksmith's search for a way to use this power without sacrificing themselves.
One idea could be a blacksmith who creates unique ninja tools with special abilities that give the wielder an edge in battle. For example, a kunai that can change its shape during flight to avoid enemy defenses.
Another interesting concept is a blacksmith who is a descendant of an ancient and powerful blacksmith clan. They could have secret techniques passed down through generations, like forging weapons with the ability to absorb chakra attacks and then release them back at the enemy.
Another possible plot is that the blacksmith, being an outsider in the Naruto world, has to deal with the cultural differences while building his harem. He might introduce new blacksmithing techniques that are not known in the Naruto world. The female characters in his harem could help him adapt to the ninja lifestyle, and in return, he could use his skills to upgrade their ninja tools. This creates a dynamic where they all grow together and face various challenges as a group.
Perhaps a fanfiction could have a plot where the blade smith has to team up with unlikely allies to create a blade that can save the world from a new, unknown threat. They have to gather rare materials from different realms in the Highschool DxD world, facing challenges and making new friends or enemies along the way. It would be a journey of self - discovery as well as a race against time to complete the blade.
One interesting blacksmith fanfiction could be about a blacksmith in a fantasy world who forges a magical weapon that changes the fate of the kingdom. Maybe the blacksmith discovers a rare ore that gives the weapon special powers.
One interesting idea could be the combination of Naruto's ninja skills with mecha technology. For example, Naruto might use his chakra to power up the mecha in unique ways. Another idea could be the new enemies that are created specifically for this mecha - Naruto concept, like robotic shinobi that require different combat strategies compared to normal foes. Also, the design of the mecha Naruto itself can be really cool, perhaps with features inspired by the Nine - Tails.
Another idea is that Aizen somehow ends up in the Naruto universe during the time of the Fourth Great Ninja War. He could try to manipulate both sides for his own gain. But Naruto, with his ability to see the good in people, might try to reason with Aizen and show him a different path. It could be a story of redemption or a great battle if Aizen refuses.
In a 'Naruto Mermaid Fanfiction', the mermaids could be in danger from an evil organization that wants to capture them for their precious scales. Naruto teams up with a mermaid princess to stop this threat and in the process, they learn about each other's worlds.
One idea could be a crossover where Naruto and Pucca end up in the same world due to a strange ninja jutsu gone wrong. Naruto is confused by Pucca's constant chasing of Garu, and he tries to help Garu escape in his own Naruto - style way.
Well, you could write a fanfiction where the Nine - Tails in Naruto has some sort of connection to a powerful entity in the Seraphim universe. This connection could lead to a series of events that require Naruto and the Seraphim characters to work together. Or perhaps a character from Seraphim is reincarnated as a ninja in the Naruto world and has to adapt to the new way of life while still having some of their Seraphim abilities.
You could also write a fanfiction about Naruto being raised by a different clan in the village. For example, if he was raised by the Uchiha clan, it would have a huge impact on his character development and relationships within the village.
One idea could be that Krampus is summoned in the Naruto world during a particularly harsh winter. He mistakes the ninja villages for places full of naughty 'children' (in this case, ninjas who have misbehaved according to some warped view). Naruto then has to convince Krampus that the ninjas are actually good at heart and are just trying to protect their homes.