Maybe the symbiote is actually a creation of an ancient enemy of the Kyuubi. When it bonds with Naruto, it initially wants to use her to get to the Kyuubi. However, as time passes, it starts to develop a sense of loyalty towards Naruto and instead helps her and the Kyuubi fight off other threats. For example, it could give Naruto the ability to transform into a new and powerful form that combines the features of the symbiote and the Kyuubi's power.
One interesting plot twist could be that the symbiote actually has a mind of its own and tries to take over Naruto completely, but the Kyuubi's chakra interferes and creates an unexpected alliance between the three. This could lead to Naruto having new and strange powers that no one has seen before.
One interesting plot twist could be that in the fanfiction, the fem Naruto in the Kyuubi suit has the ability to communicate with the spirits of past Jinchuriki. This gives her an edge in battles as she can learn from their experiences. Another twist might be that the suit itself has a mind of its own and sometimes takes over to protect Naruto, even if it means going against her own wishes temporarily.
Well, in some 'Naruto corrupted by fem Kyuubi' fanfiction, a great plot twist could be that the fem Kyuubi is actually a part of a larger prophecy that Naruto was unaware of. The corruption she brings is a necessary step in fulfilling this prophecy. It could also be that the fem Kyuubi's true form is not what it seems, and when Naruto discovers this, it completely changes his perception of her and their relationship. For instance, she might be a reincarnation of an ancient and powerful being, and her 'corruption' of Naruto is a way to awaken his latent powers related to that ancient being.
The symbiote could make Naruto's relationships more complicated. For her friends, they might be initially scared or distrustful of the new - looking Naruto with the symbiote. For example, Sasuke might see her as a threat now. But as they learn that the symbiote is actually helping Naruto control the Kyuubi, they might slowly accept it.
One interesting plot twist could be that the sealing process goes wrong in a completely unexpected way. Maybe instead of just sealing the Kyuubi's chakra, Naruto accidentally fuses with it on a spiritual level, giving him new and uncontrollable powers at first. Another could be that during the sealing, a dark entity from another dimension tries to interfere and take over the Kyuubi's power within Naruto. It would add a whole new layer of conflict and mystery to the fanfiction. Also, perhaps the seal actually has a hidden purpose that was set by an ancient force long ago, and as Naruto discovers bits of this, it changes his understanding of his role as the Kyuubi's host.
Well, there's a fanfiction called 'Kyuubi's True Form'. The plot twist here is that the Kyuubi has a completely different true form than what was expected, and this form has different powers and a different relationship with Naruto. Also, in 'Naruto's Lost Kyuubi', Naruto suddenly loses the Kyuubi for an unexpected reason, like a time - travel accident, and he has to find a way to get it back without his usual power source.
One popular plot twist could be that as a female Naruto born as the Kyuubi, she has an even stronger connection to nature. Maybe she can communicate with the forest spirits in a way Naruto in the original couldn't. This could lead to new alliances and powers. Another could be that her female identity causes different reactions from the villagers. Instead of just fear and hatred like Naruto faced, there could be a mix of awe and jealousy, which would change her relationships and how she grows up.
Well, a cool plot twist might be that Fem Naruto has a secret lineage related to Madara's own clan. Maybe she discovers this and it makes Madara question his long - held beliefs and grudges. Or perhaps in the fanfiction, Fem Naruto is able to use a new form of ninjutsu that is a combination of Uzumaki and Uchiha techniques, which she uses to outwit Madara in a crucial battle. It could also be that Madara's real goal was to actually find a successor like Fem Naruto all along and his actions were just a test to see if she was worthy.
A great plot twist could be that fem Arthur is actually related to one of the hidden clans in the Naruto world. This revelation changes the way Naruto and his friends view her and also adds a whole new layer of mystery to her character.
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.
One possible plot twist could be that the fem Kyuubi, instead of being an enemy, turns out to be an ally of Naruto the mercenary. Maybe she has her own motives for working with him, like she wants to use his skills to gain freedom from some other powerful force that has been controlling her.