How about a weapon that is powered by the user's emotions? When the character is angry, the weapon becomes stronger and more powerful. It could be a bracelet that glows and releases energy based on the intensity of the user's feelings. This would add an interesting psychological aspect to the battles in your fanfiction.
In such a fanfiction, a whip made of chakra - resistant material could be made. This whip could be used to disarm opponents or even bind them. The creator could also come up with a gauntlet that has the ability to absorb and redirect an opponent's chakra attacks. It would work by having special seals on the gauntlet that trap the incoming chakra and then send it back towards the attacker.
In some fanfictions, Naruto crosses over into a world of ancient mythology. He meets gods and monsters from different mythologies like Greek, Norse or Egyptian. His ninja powers are put to the test as he tries to navigate this new and dangerous world. It's an interesting idea to see how he fares against such powerful and otherworldly beings.
In some fanfictions, Kurama can take a human form and interact with Naruto in a more 'human' way. For instance, they could go shopping together in the Hidden Leaf Village.
One unique idea is the exploration of their artistic views. Deidara is obsessed with his explosive art, while Naruto has his own idea of what it means to be strong and leave a mark. Some fanfics play with the idea of them sharing and comparing these views, like in 'The Art of Bonding' where they have a series of conversations about their respective 'arts'.
Another unique idea is to have a Hyuga character who completely rejects the traditional ways of the clan. Instead of following the strict hierarchy and training methods, they venture out on their own path, maybe even teaming up with outcasts from other clans. This creates a whole new dynamic and story as they face opposition from their own clan while trying to prove that their new way can be just as effective or even better.
One unique idea is to have the chunin exams take place in a completely different setting, like an underwater city. Another could be making the main characters lose their powers temporarily during the chunin exams, adding a whole new level of challenge. And there's also the idea of having non - ninja characters participate in the chunin exams in a 'naruto chunin fanfiction'.
One idea could be that Naruto gets a ponytail after a magical encounter. For example, he meets a spirit who gives him a new hairstyle as a gift, but the ponytail has special powers like it can sense danger. So, when there's an enemy approaching, the ponytail glows or something.
A jutsu that allows the user to transform their chakra into a liquid form that can seep into the ground and then resurface anywhere within a certain radius as a surprise attack. Another could be a jutsu that uses sound waves to disrupt an opponent's chakra flow, like a high - pitched noise that throws off their concentration and ability to perform jutsus.
Some 'Naruto Kaguya fanfiction' has the unique idea of Kaguya having a different origin story. Maybe she was forced into her actions by some unknown power. Also, there are fanfictions where Kaguya imparts her ancient knowledge to the younger generation in the Naruto world, which is really interesting. It shows a side of her that could be a mentor rather than a destroyer. This changes the whole dynamic of how we see her character.
Well, often in such fanfics, you might find a weapon that is a combination of different elements. For example, a spear that has the power of both fire and wind chakra. It can not only cut through opponents like a normal spear but also send out blasts of combined fire - wind jutsu. There could also be a shield made of special chakra - resistant metal which can deflect even the most powerful ninjutsu attacks.