In a fanfiction, you could explore what would happen if an ordinary citizen in the Naruto world got their hands on explosive tags. This person has no ninja training but has to figure out how to use the tags to protect their family from a group of bandits. It could be a story of self - discovery and unexpected heroism.
One idea could be a story where a young ninja discovers a new way to use explosive tags in combat that no one has thought of before. Maybe they can combine them with a special jutsu to create a super - powerful attack.
One idea could be a young ninja discovers a new form of the Explosive Fist technique. He then has to train hard to master it while facing challenges from rival clans who want to steal the technique. Another could be a story where a character uses the Explosive Fist to save a village from a powerful enemy that no one else could defeat. Maybe they combine it with other jutsus in unique ways.
To start, think about the purpose of your fanfiction. Are you writing it to explore a new relationship between characters? Or maybe to show a different side of the Naruto world. Once you have that in mind, start with a hook. For instance, 'The night was dark as the explosive tags glowed ominously in the hands of the unknown figure. Who was this person and what were they planning with those dangerous tags?' Then build your story from there, introducing characters, settings, and conflicts gradually.
One interesting Naruto explosive clay fanfiction could be about a new character who has a unique way of using explosive clay that surprises everyone in the Naruto universe. Maybe they use it to create beautiful yet powerful sculptures that can be used in battle.
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.
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.
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.