In this kind of fanfiction, Naruto might gain even more incredible power. He could have enhanced physical abilities like super strength and speed, as well as new jutsu related to the bijuu's nature. For example, if he becomes the Nine - Tails bijuu, he might be able to use the Tailed Beast Ball more effectively and on a larger scale. His appearance might also change, perhaps taking on some of the characteristics of the bijuu, like fox - like features if it's the Nine - Tails. This could lead to new relationships and dynamics with other characters. His friends might be in awe of his new power, while his enemies would be even more fearful.
If Naruto becomes a new bijuu in fanfiction, he might gain even more power. He could have unique abilities that are a combination of his existing skills and the power of a bijuu. For example, his already strong chakra control might be enhanced, allowing him to perform more powerful jutsus with less effort. Also, his connection to other characters would change. His friends might view him with a mix of awe and concern, as they would have to adapt to this new, more powerful Naruto.
Well, perhaps in the 'naruto becomes a wolf bijuu fanfiction', Naruto is transformed into a wolf - like Bijuu form due to some dark magic or a strange experiment gone wrong. His friends initially don't recognize him, and he has to struggle to control this new form while also trying to show them it's still him. He may learn new jutsus that are unique to this wolf - bijuu form and use them to protect the village from new threats that are too powerful for his normal form.
It could be Orochimaru. He's always interested in strange and powerful forms, and Naruto's new wolf - bijuu form would be something he'd want to study or even control for his own evil purposes.
The Akatsuki could also be the main antagonists. Since they are after the bijuu, they would surely target Naruto once he has this special relationship with them. They might try more elaborate schemes to capture the bijuu from Naruto and his 'harem' of sorts.
In this fanfiction, Naruto could gain the ability to communicate with the bijuu on a deeper level. As a result, they start to see him as not just a jinchuriki but as a friend or even more. The plot could involve Naruto trying to protect the bijuu from threats while also dealing with the complex relationships that form within the 'harem' situation.
In fanfiction, if Naruto is half - bijuu, it could mean he has enhanced physical abilities. For example, he might have greater strength and speed like a full - bijuu. His chakra reserves could also be much larger, allowing him to perform more powerful jutsu for longer periods.
Well, one possible plot could be that Naruto is somehow magically transformed into a dragon bijuu. He might then have to learn to control his new form and powers. Maybe he'd face off against enemies who are both intrigued and threatened by his new state. His friends could be initially shocked but then try to help him adapt.
It could be an ancient sage. In Naruto lore, there are sages who have had a great influence on the world. An ancient sage could be the one who created or fathered the bijuu. Maybe he had some great power and knowledge that allowed him to bring the bijuu into existence.
In many Naruto fanfictions, if Naruto has three bijuu, they could be the Nine - Tails (Kurama) which he already has in the original series, perhaps the Eight - Tails (Gyuki) as it has a connection with Naruto's world and Akatsuki's hunt for the bijuu. And another could be the Three - Tails (Isobu). However, it really depends on the specific plot and creative direction of the fanfiction.