The plot could center around Naruto's internal struggle as he becomes a tailed beast. He may initially embrace the power, thinking it will make him stronger to protect his loved ones. But then he realizes that he's losing himself. He could have visions or dreams of the other tailed beasts, who either try to guide him or lead him astray. His relationship with Sasuke might also be affected, as Sasuke might see Naruto as a threat now, and Naruto has to prove that he can still be trusted even in his new form.
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.
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.
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.
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.
Perhaps the tailed beast inside Sakura has a mind of its own. It could communicate with her and offer her different perspectives on the ninja world. This could lead to Sakura making choices that are different from what her friends expect. For example, she might try to broker peace with an enemy village because the tailed beast has shown her the futility of war. The story could also involve her training with Kakashi or other experienced ninjas to better understand and use the tailed beast's power.
A plot could revolve around a tailed beast that has been corrupted by an unknown force. Naruto and his friends then have to find a way to purify the beast without killing it. This would require them to understand the nature of the tailed beasts more deeply. In the process, they might discover new secrets about the tailed beasts' origins and their connection to the ninja world. It could also lead to new alliances and enemies. There could be intense battles and emotional moments as they try to save the corrupted tailed beast.
The plot could involve Dante, from his own universe, being suddenly transported to the world of the Nine - Tailed Beast. He might initially be seen as a threat, leading to battles. But then, they could discover they have a common enemy and team up. Maybe Dante's combat skills and the Nine - Tailed Beast's power could be combined in an epic showdown against a greater evil.
One idea could be exploring the ten - tailed beast's origin in more depth. Maybe it has a hidden past or a connection to an ancient civilization that has been long forgotten in the Naruto universe.
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.
A possible plot could involve Naruto being able to enter the minds of the tailed beasts more freely and vividly. In this state, he experiences their memories as if they were his own. He witnesses the creation of the tailed beasts, the wars they were involved in, and the pain they've endured. This newfound understanding makes him determined to create a new world where the tailed beasts are not feared but respected. So he starts a movement among the ninja villages, using his influence and the power of his bond with the tailed beasts to change people's perceptions.