There could be a powerful and jealous ninja from another village. This ninja wants the power of the Shukaku for himself. He sees the relationship between Naruto and the female Shukaku as an obstacle to his goal, so he devises plans to separate them and capture the female Shukaku, making him a major antagonist in the fanfiction.
Their relationship could start with mistrust. Since Shukaku is a tailed beast, Naruto might be cautious. But as they interact more, like when they are forced to fight together against a common foe, they start to respect each other.
A possible plot is that the female Shukaku is captured by an evil organization. Naruto, being the hero he is, sets out on a rescue mission. Along the way, he discovers more about the female Shukaku's past and her connection to the history of the ninja world. This could involve ancient secrets and prophecies related to the tailed beasts.
One idea could be that the female Shukaku has been living in hiding among the Leaf Village. Naruto discovers her secret and they form an unlikely bond as they train together to control her Shukaku powers. This bond then turns into a deep friendship that might lead to more as they face various threats to the village.
One interesting storyline could be about the relationship between the Shukaku and Naruto evolving in a unique way. For example, the Shukaku might teach Naruto some ancient and powerful jutsu that was once known only to the tailed beasts. This could give Naruto an edge in battles and also deepen their bond.
Naruto would win. He has a lot of experience fighting powerful opponents and has developed many powerful jutsu over time. He also has the willpower and the ability to adapt in battle. Shukaku is strong, but Naruto's intelligence and his ability to think on his feet would give him the edge.
Kakashi can also be involved in some shukaku naruto fanfic. He could be Naruto's mentor in understanding the Shukaku better. He has a wealth of knowledge about the ninja world and could offer advice to Naruto on how to handle the power of the Shukaku and any potential risks associated with it.
A plot might involve Shukaku being controlled by an evil force and Naruto has to fight it to save the village. Naruto starts by trying to reason with Shukaku, but when that fails, he has no choice but to engage in a fierce battle. He might call upon the help of his friends like Sasuke or Sakura at some point to create a strategy to defeat Shukaku.
One popular theme is the relationship between Gaara and Fem Shukaku. It could explore how they interact in a world where Shukaku has a female form. Another theme might be the power dynamics. Since Shukaku is a powerful entity, in a fanfiction, it could show how Fem Shukaku's power affects Gaara's life and his role in the village. Also, themes of friendship and mutual understanding often come up. As Gaara has had a difficult past, Fem Shukaku could potentially be a source of newfound support and a different kind of connection for him.
The canon relationship between Gaara and Shukaku is very much about Gaara suppressing the power of Shukaku. However, in fanfiction with Fem Shukaku, their relationship could be based on mutual growth. Fem Shukaku could teach Gaara different ways to use and understand his power. Their interactions might not be as combative as in the original story. Instead, they could be more about sharing knowledge and experiences. For instance, Fem Shukaku might have a different perspective on the use of chakra that she could share with Gaara, leading to a new dimension in their relationship.