The antagonists could be a secret organization within the Hidden Leaf Village itself. They are afraid that if Kurama is freed, it will bring destruction to the village. They think Naruto is being reckless and they will do anything to stop him, including framing him for crimes he didn't commit and turning his friends against him, all in an effort to keep Kurama sealed.
In this kind of fanfiction, one possible plot could be that Naruto discovers a hidden ancient jutsu. With great determination and after many trials, he manages to use this jutsu to break the seal that binds Kurama. Then, Kurama, instead of being hostile as expected, actually thanks Naruto for freeing it. They form an even deeper bond and decide to work together to protect the Hidden Leaf Village from a new and powerful enemy that is approaching.
The main characters are Naruto and Kurama, of course. Naruto is the protagonist who wants to free Kurama. Kurama is the nine - tailed fox that is sealed within Naruto and is the one being freed.
Well, in some fanfics, the story might start with Naruto getting a premonition or some kind of sign that Kurama is suffering under the seal. So, he decides to free Kurama. He then embarks on a journey to find a way to break the seal. Along the way, he meets various characters from the Naruto universe who either help or hinder his progress. Once he finally frees Kurama, they team up to face a greater threat to the ninja world that neither could have faced alone.
Maybe there are powerful dark sorcerers. These sorcerers could be trying to capture Naruto for their own evil purposes, like using his power to open a portal to a dark dimension or something, and Kurama has to protect him from them.
In certain 'naruto and kurama team up fanfiction', there are antagonists from other hidden villages. They might be jealous of the power that Naruto and Kurama possess together and want to take it for themselves. So, they plot to kidnap Naruto and force Kurama out of him, which sets off the conflict between them.
Sasuke could potentially be an antagonist. Even though they were friends before, Sasuke might see Naruto's new fused power as a threat to his own goals or ideals. He could challenge Naruto, thinking that Naruto's power is getting out of hand. Another could be some remnants of the Akatsuki. They might be interested in Naruto's new power and try to either steal it or use it for their own evil purposes.
Sakura could be a great help. Her medical skills could come in handy if Naruto gets injured during his battles or difficult encounters while trying to free the tailed beasts. Also, her combat skills have improved over time, so she could fight alongside Naruto.
Well, Naruto is the key character as he is the one who frees Jenova. Jenova herself is a central character in this fanfiction concept. Other characters from the Naruto universe could also play roles. For example, Sakura might be concerned about Naruto's actions and try to talk some sense into him. Also, there could be some original characters created for this fanfiction. Maybe a wise old hermit in the realm where Jenova was imprisoned who warns Naruto about the consequences of freeing her, but Naruto, being stubborn, doesn't listen at first.
The plot of a 'naruto frees kyuubi fanfiction' might involve an external threat to the village. This threat is so great that Naruto realizes he needs the full power of the Kyuubi, not just the limited access he has through the seal. So, he starts to work on breaking the seal. Along the way, he has to face opposition from some in the village who fear the unleashed Kyuubi. But Naruto is determined. He finally frees the Kyuubi and together they defeat the external threat. After that, the Kyuubi decides to stay with Naruto as a friend and ally, seeing the true nature of the young ninja.
One possible plot could be that Naruto, with his strong sense of justice and determination, discovers Jenova being held captive in some strange, other - worldly prison. Using his ninja skills like shadow clones and rasengan, he battles the guards or forces holding Jenova. He then frees her, not fully understanding the implications. Jenova, being a complex and powerful entity, initially sees Naruto as a tool but eventually starts to respect his unwavering will.