Well, in the 'kyuubi gets bigger' fanfiction, it could be related to a dark magic or an ancient curse. For example, an evil sorcerer might cast a spell on the kyuubi, making it grow uncontrollably. As it gets bigger, it could start to disrupt the balance of nature in the ninja world. And Naruto and his friends would have to find a way to break the spell and stop the kyuubi from causing more chaos.
Obviously Naruto would be a main character. Since he has the kyuubi sealed within him, any change in the kyuubi's state would directly affect him. He would be in the center of the story, trying to deal with the kyuubi getting bigger.
There should be no such fanfiction as it involves extremely inappropriate and unethical content. Fanfiction should focus on positive, creative and ethical storytelling, not on such offensive and immoral scenarios.
Yes, there might be such fanfictions in the vast world of Naruto fanfiction. However, this is a rather unusual and non - canonical concept. Some fanfiction platforms may have works exploring such out - of - the - box ideas, but it's not part of the official Naruto storyline.
In such a fanfiction, Naruto would likely face extreme danger. The extraction of the Kyuubi would leave him in a very weak state, perhaps near death. His body has been so intertwined with the Kyuubi's chakra for so long that suddenly losing it could cause his systems to shut down. Also, his ninja abilities that were enhanced by the Kyuubi would disappear, and he'd have to relearn how to fight without that power boost.
In some fanfictions, Naruto might obtain the Kyuubi's bloodline through a special ritual. This could be a ceremony performed by a powerful shinobi or a sage who has the knowledge to transfer the essence of the Kyuubi's power into Naruto's genetic makeup, thus giving him the bloodline.
In some fanfiction concepts, it might be due to a unique chakra interaction. Maybe the Kyuubi's powerful and ancient chakra somehow triggers a latent ability within Naruto that results in the manifestation of the Rinnegan. It could also be part of some kind of prophecy or destiny within the fan - created world of the story.
Well, in some fanfictions, Naruto might obtain the sword through a special bond or trial. Maybe Kyuubi decides that Naruto needs this weapon to face a new and powerful enemy. So it could be that Naruto proves his worth in some way, like showing great courage or a new level of control over his chakra, and as a result, Kyuubi gives him the sword.
In some fan theories, it could be due to a unique chakra interaction. Maybe the Kyuubi's powerful chakra somehow mutates Naruto's own ocular abilities and triggers the emergence of the Sharingan. But this is just a wild fanfiction idea.
If the fem Kyuubi gets control of Naruto in the fanfiction, one consequence could be that Naruto's relationships with his friends might be strained. His friends might not trust him anymore as they notice the change in his behavior under the Kyuubi's control. Another possibility is that Naruto could be used to cause chaos in the village. Since he's very powerful, the fem Kyuubi could use him to carry out her own plans, which might be harmful to the village and its people.