Well, a common theme in such fanfictions is the chaos that ensues. The release of the Kyuubi is a major event. Villages could be destroyed, and Naruto has to learn to control this power quickly. Some fanfictions might focus on the training he undertakes to master the Kyuubi's power, perhaps with the help of some unexpected mentors.
One possible consequence could be chaos in the ninja world. With Naruto, a major force for peace, dead and the Kyuubi released, other villages might see an opportunity to gain power or territory. Villages that once relied on Naruto's protection would be vulnerable. Also, the Kyuubi, being a powerful and wild force, could start causing destruction on its own, without Naruto to control it.
Madara Uchiha could also be a main antagonist in this fanfiction. He has grand plans for the ninja world and the released Kyuubi could be a key part of his scheme. Since Naruto is dead and can't oppose him, he might try to use the Kyuubi to reshape the world according to his vision. He has the power and the ambition to be a major threat in this situation.
Obviously, Naruto is the central character as it's about the Kyuubi being released from him. Sasuke also often plays a crucial role. He might be concerned about Naruto's new state, or he could be one of the characters trying to either help or stop Naruto depending on the plot.
Well, in such fanfictions, Naruto often relies on his ninja skills, along with the support of his friends and mentors. Sometimes, he also taps into hidden reserves of chakra and unlocks special abilities to battle the Kyuubi.
It could be an original character created by the fanfiction author. This character might be a descendant of the original Kyuubi, but with a female form and her own distinct personality.
The Kyuubi often adds an element of mystery and power to Naruto. In fanfictions, this can translate into his relationship with Hinata in various ways. For example, the Kyuubi's power might be seen as something that makes Naruto more worthy of Hinata in the eyes of some characters in the story. Or, the Kyuubi could have a will of its own and actually push Naruto towards Hinata, seeing her as a good influence on Naruto. This can lead to some very unique and engaging story arcs within the fanfiction.
In the fanfiction, Kyuubi could start off as a very self - centered character, only interested in its own power and freedom. But through interactions with Kurama and facing common challenges, it starts to develop a sense of responsibility towards others. Kurama, on the other hand, is initially very reserved. However, as it spends more time with Kyuubi, it begins to open up more and show a more emotional side, like getting angry more easily when Kyuubi makes a particularly dangerous or stupid move.
The fem Kyuubi could have more delicate features compared to the traditional, more menacing Kyuubi. For example, her eyes might be more almond - shaped and have a softer glow. Her fur could also be a different shade, perhaps a lighter red or with some kind of pattern on it.
Maybe it's because Naruto has some sort of special connection or resemblance to the Kyuubi that Kushina notices. For example, Naruto might display some of the Kyuubi's power characteristics in an obvious way that makes Kushina jump to that conclusion.