Well, perhaps a plotline where Naruto, reborn as the Kyuubi, has to deal with the reactions of his friends and former enemies. His friends might be initially scared or distrustful, while his enemies might see him as an even greater threat. This could lead to a lot of conflict and character development as Naruto tries to prove that he's still the same person deep down.
One interesting plotline could be Naruto gaining new and unique powers from the merge. For example, he might be able to access a special form of chakra that combines his own with the Kyuubi's, allowing him to perform super - strong jutsu. Another could be how his relationships with his friends and allies change. Maybe some are initially afraid of his new power, but he has to prove that he's still the same Naruto at heart.
Some fanfictions focus on the idea of a different kind of seal being used on the Kyuubi. Maybe a more humane or symbiotic seal that allows Naruto to fully utilize the Kyuubi's power without the negative side effects. This can lead to a whole new Naruto, one who is more in control and perhaps even more powerful than in the original story.
One interesting plotline could be Naruto gaining new and unique powers from the fusion. For example, he might be able to access a new form of chakra that combines his own with the Kyuubi's. This could make him even more powerful in battles against tough opponents like Pain or Madara.
One plotline could be the Kyuubi teaching Naruto some ancient and powerful ninja techniques that were lost over time. Since the Kyuubi has been around for a very long time, it has a wealth of knowledge. Another could be Naruto and the Kyuubi having to team up to face a new, powerful enemy that neither of them could defeat alone.
Another interesting plotline is that the Kyuubi eye gives Naruto the ability to communicate with the spirits of past Jinchuriki. They teach him ancient techniques and give him insights into the true nature of the tailed beasts. This new knowledge makes him an even more formidable force, but also makes him a target for those who fear the power of the Jinchuriki.
One possible plotline could be the Kyuubi teaching Naruto unique and powerful jutsu that he can't learn from his normal training. Another could be the Kyuubi helping Naruto understand his own heritage and the secrets of the Uzumaki clan in a more in - depth way. Maybe also the Kyuubi guiding Naruto to control his chakra better in a way that no one else could teach him.
One interesting plotline could be Naruto and Kyuubi learning to truly understand and trust each other on a deeper level. They might have to face a common enemy that forces them to combine their powers in a new and unique way.
One interesting plotline could be Naruto and the good Kyuubi's merged form facing off against an ancient evil that threatens the entire ninja world. Their combined power gives them unique abilities to combat this threat.
Well, in some versions of 'naruto gets kyuubi soul fanfiction', Naruto may have to deal with the Kyuubi's memories. These memories could be of battles long ago or the Kyuubi's past experiences. This could lead to Naruto having a deeper understanding of the world of chakra and the history of the tailed beasts. Also, there might be a plot where Naruto has to convince the Kyuubi soul to fully cooperate with him, rather than just being a power source.
One interesting plotline could be the Kyuubi teaching Naruto unique and powerful jutsu that are not part of the regular ninja training. For example, it might teach him a special form of chakra control that gives him an edge in battles. Another plotline could be the Kyuubi helping Naruto understand the history of the ninja world from its own ancient perspective, which in turn helps Naruto make wiser decisions in his journey to protect the village.