Obviously, Naruto is the main character as he is the one releasing the Kyuubi. Sasuke could also be involved, perhaps as an ally or an antagonist depending on the plot of the fanfiction. Sakura might be there too, trying to support Naruto in controlling the Kyuubi's power or dealing with the aftermath.
One interesting scenario could be Naruto learning to control the Kyuubi's power in a unique way that wasn't shown in the original series. For example, he might use it to create new jutsu that combine his own skills with the Kyuubi's raw power.
In some fanfictions, the immediate consequence could be destruction. Buildings might be razed and many shinobi could be injured or killed. The village would be in chaos as they try to either contain Naruto or the Kyuubi's power. There could also be a shift in the power dynamics, with some seeing Naruto as a threat and others trying to find a way to calm him down and reseal the Kyuubi.
Well, in this fanfiction, when Naruto releases the Kyuubi fuuinjutsu, it could completely change the power dynamics in the Naruto universe. His friends might be both in awe and a bit scared of his newfound strength. Naruto would have to learn to control this power quickly, or it could end up causing more harm than good. He might also face new challenges as other villages view him as an even greater threat now.
One well - written female Kyuubi character could be the one who is depicted as having a complex personality. For example, she might be fierce in battle but also show vulnerability when it comes to her relationship with Naruto. Another could be a female Kyuubi who has a strong sense of justice, despite her origin as a powerful and sometimes feared entity. This kind of character adds depth to the fanfiction.
One popular theme could be the power dynamics between Naruto and the female Kyuubi. Since Naruto has the ability to control or interact with the Kyuubi's power, in a fanfiction with a female Kyuubi, it can lead to some interesting scenarios of trust - building. Another theme might be a romantic relationship. The idea of Naruto, a human ninja, falling in love with a powerful and somewhat otherworldly female Kyuubi can be a draw for many readers. Also, stories about them teaming up against common enemies while exploring their unique bond are also quite common.
It could be Orochimaru. He's always been interested in power and the secrets of the Kyuubi. With Naruto releasing the Kyuubi fuuinjutsu, Orochimaru might see this as an opportunity to either steal Naruto's power or use him for his own evil experiments.
The release of the Kyuubi could also attract other threats. Enemies from other villages or dark forces might see this as an opportunity to either steal the Kyuubi's power or take advantage of the chaos in the Hidden Leaf. Naruto, while dealing with the Kyuubi's power, would also have to face these external threats. His journey could involve learning to master the Kyuubi's power in a way that protects the village instead of endangering it.
In Naruto fanfiction, the combination of Fuinjutsu and blacksmithing can be really interesting. Fuinjutsu is all about sealing techniques, and when combined with blacksmithing, one can imagine creating weapons with sealed powers. For example, a sword that has a sealing jutsu inscribed on it to trap the opponent's chakra when it strikes. It adds a whole new layer of creativity to the Naruto universe.
In such fanfiction, the blacksmith could be a master of both metal and seals. They might create a chain with Fuinjutsu that can bind an opponent's chakra. This would be a great addition to a ninja's arsenal. They could also work on larger items like gates with Fuinjutsu seals to protect a village or a secret location.