An interesting original character for a fanfiction could be a time - traveling ninja. This character could come from the future and have knowledge of events that are yet to happen in the Naruto world. They could try to change certain events for the better or worse, depending on their motives. Another option is a character who is a hybrid of a human and a summon animal. This would give them unique abilities such as enhanced strength, speed, or the ability to communicate with other summon animals more easily. A third idea is an original character who is a master of disguise, able to change their appearance and chakra signature completely, and they use this ability to infiltrate enemy organizations.
One idea could be a story where Naruto discovers a new hidden power within him that has a connection to an ancient and forgotten ninja clan. He then has to learn to control this power while facing off against a new, powerful enemy that is specifically targeting him because of this new ability.
One idea could be a time - travel plot where Naruto goes back in time to the era of the First Hokage. This would allow him to change some of the events that led to the long - standing conflicts in the shinobi world. He could also learn some of the ancient and lost jutsus from that time. Another plot could be that Naruto discovers a hidden ninja village that has been isolated for centuries and has unique powers and customs. He then has to figure out how to incorporate this new village into the existing shinobi world order.
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.
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.