It could be an original character. For instance, a young girl from a small, forgotten ninja clan who has a natural affinity for the tailed beasts. Maybe she has a special kekkei genkai that allows her to sense their emotions from afar, and she embarks on a journey to conquer the tailed beasts to prove her clan's worth.
Naruto himself could still be the main character in a different kind of tailed - beast - conquest story. What if there was a new threat that was causing the tailed beasts to go berserk? Naruto, already having a connection with them, would have to find a new way to fully conquer their power and bring them back to a state of harmony, this time facing challenges he has never encountered before.
Sasuke could also be a main character in a tailed beast conquest fanfiction. After his redemption arc, he could decide to take on the task of taming the tailed beasts to atone for his past sins. He has the power and the determination, and his Sharingan could play an interesting role in dealing with the powerful entities that are the tailed beasts.
It could be Orochimaru. He has always been interested in power and the secrets of the tailed beasts. He might see Naruto's transformation into a tailed beast as an opportunity to study and use that power for his own evil purposes. He could try to manipulate Naruto in his new state, perhaps by using genjutsu or other underhanded tactics.
Well, Naruto himself might still be a main character as he has a strong connection to the tailed beasts in the original series. He could be the one trying to understand and deal with the 10 - tailed beast in new ways.
One plot idea could be a young and inexperienced ninja discovers they have a unique connection to the tailed beasts that no one else has. For example, they can communicate with the beasts on a deeper level, not just the usual 'host' relationship. This could lead to them trying to unite the tailed beasts in a peaceful way, against all odds and the mistrust of the different ninja villages.
He could be experimented on by rogue ninja scientists. These scientists might have been researching ways to create super - powerful beings and see Naruto as the perfect test subject. They use some forbidden jutsu or strange technology to try and force the creation of an 11 - tailed form within Naruto.
Iroha. She's not a very prominent character in the main Naruto story, but in fanfiction, she could be given a special ability that makes her crucial in dealing with the ten - tailed beast. Maybe she has some kind of ancient bloodline power that can either calm the beast or weaken it in a unique way.
In this fanfiction, the plot might involve Naruto being experimented on by some dark force. This experiment causes him to transform into a tailed beast in a way that's different from his normal jinchuriki forms. As he changes, he starts to view the world from a more primal perspective. He might even break away from the Hidden Leaf Village and the story could be about his journey to regain his sanity and his place among his friends and the village.
Another possibility is that one of the tailed beasts could be the main character. Maybe it's the nine - tailed fox, for example. The story could be told from its perspective as it tries to understand its own origin and the role of the mother of the tailed beasts in its creation and existence. It could be a story of self - discovery for the tailed beast, with the mother being a key part of that discovery.
One possibility is that Naruto would initially be confused and struggle to control his new form. He might accidentally cause destruction in the village as he tries to figure out his new powers. But with the help of his friends, like Sasuke and Sakura, he could start to learn to harness the tailed beast's abilities for good, perhaps even developing new jutsu.
One possible plot twist could be that Naruto's new form as a tailed beast gives him the ability to communicate with all the tailed beasts simultaneously. This could lead to him uniting them in a way that no one has ever seen before, and they might work together to bring about a new era of peace in the ninja world.
I think 'naruto ten tailed beast fanfiction' can be really interesting. It often explores the power and mystery of the ten - tailed beast in unique ways. Some fanfictions might focus on how the characters interact with this powerful being, like Naruto's potential new jutsus or his changed relationships due to the ten - tailed beast's influence.