It could also be that the kyuubi gets more power as a result of its own self - discovery. It delves deep into its own nature and finds latent powers that it never knew it had. For example, it might find out that it has the ability to manipulate time on a small scale. This new power would change the way it interacts with the world around it and the other characters in the fanfiction.
Perhaps the kyuubi gets more power through a pact with another powerful being. It could be a spirit from a different realm. In this fanfiction, they make an agreement where the kyuubi gains new powers in exchange for some future service. This would lead to the kyuubi having to face moral dilemmas as it tries to figure out how to use its new - found powers while fulfilling its side of the pact.
One possible plot could be that the kyuubi discovers a hidden ancient power source. Maybe it's a long - forgotten magic crystal or a special energy field. This new power source could enhance its existing abilities, like making its fire attacks more intense or its speed much faster.
Obviously, the kyuubi itself is the central character. But there could also be characters like the shinobi who are either trying to control the kyuubi's new power or are afraid of it. Maybe there's a young and brave ninja who wants to befriend the kyuubi and help it use its power for good.
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.
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.
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.
In this fanfiction, perhaps Naruto's physical abilities are enhanced as the Kyuubi's kit. He might be able to move faster, jump higher, and his strength could be superhuman. Also, his relationship with Sasuke could change. Sasuke might see Naruto as an even greater threat or, on the other hand, an ally he needs to understand better. And Naruto could start to have a different understanding of his own identity and place in the ninja world.
One plot twist could be that the 'pregnancy' actually turns out to be a way for Kyuubi to transfer its remaining power to Naruto in a more permanent and extreme way. Instead of just lending power, it's like a total fusion of their essences. Another could be that the 'pregnancy' is a ruse by an enemy who has manipulated Naruto's connection with Kyuubi to make it seem like this is happening, when in fact they are trying to steal the power that would be created from such a union.
In some fanfictions, the Kyuubi's power could be depicted as more spiritual. Instead of just raw chakra, it could have elements of ancient wisdom and spiritual energy that Naruto can tap into. For example, Naruto might be able to use the Kyuubi's power to communicate with the spirits of the past. Another way is to show the Kyuubi's power as having different forms. Maybe there's a 'benevolent' form of the Kyuubi's power that Naruto can access for healing or peace - making purposes, as opposed to the more destructive form we usually see.
Well, perhaps the fanfiction could start with Naruto feeling that there is something wrong with his connection to the Kyuubi. He then embarks on a journey of self - discovery. Along the way, he meets some mysterious characters who give him clues on how to break the 'kyuubi lemon', which might refer to some sort of blockage or negative aspect in their relationship. As he progresses, he gradually overcomes these issues and in the end, he has a much stronger and more harmonious bond with the Kyuubi, and his power increases significantly. This plot could also involve some emotional turmoil for Naruto as he battles with his inner self and tries to understand the true nature of his relationship with the Kyuubi.
One possible plot point could be how Naruto discovers his true mother is the female Kyuubi. Maybe he has some strange powers or abilities that start to manifest and he realizes they are related to his mother. Another could be the reaction of the village. Since the Kyuubi has always been seen as a threat, they might be very wary or even hostile towards Naruto at first. Also, the relationship between Naruto and the female Kyuubi - mother could be explored, like how she imparts her wisdom and power to him.
Perhaps a plot twist where the seal on the Kyuubi was never complete. And as Naruto grows stronger, parts of the Kyuubi's true form start to leak out in unexpected ways. This could lead to Naruto having to struggle not only with controlling the Kyuubi's power but also with hiding this new development from his friends and enemies alike, adding a whole new layer of tension to the story.