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.
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.
His relationship with Hinata might be affected. She might be a bit more worried about him at first, but also see him as even more of a hero for having to endure this new transformation. With his friends like Shikamaru and Choji, they might be more cautious around him initially, but still be there to support him. They would have to get used to his new state and the potential danger it could bring.
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.
Perhaps he wakes up to find that the laboratory is in the middle of a big experiment gone wrong. There are strange creatures or phenomena around him, and he has to figure out how to escape and also stop whatever bad thing is about to happen. He might use his Pokémon knowledge and skills to get through this tough situation.
In the fanfiction, when Ash wakes up in the room, he might discover that he has new, unexpected powers. The room could be a sort of testing ground for him. He'll have to learn to control these powers while also dealing with the mysteries of the room, like hidden doors and cryptic messages on the walls.
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 possible plot could be that the OC wakes up in the world of Hayley Marshall and has to quickly adapt to the supernatural elements. Maybe they first struggle to understand their new powers, if they have any. They could end up becoming friends with Hayley and helping her in her battles against her enemies. For example, they might assist in protecting the werewolf pack from a new threat in the area.