Some plotlines in Naruto Nine Tails Possession Fanfiction include Naruto using the Nine Tails' power to transform in unique ways. For instance, he might be able to take on a partial Nine Tails form that gives him enhanced speed and strength without fully transforming. Another is that the Nine Tails' presence in Naruto starts to affect the natural energy around him, leading to some interesting interactions with sage mode. And then there's the idea that Naruto's connection to the Nine Tails attracts the attention of other powerful entities, either from within the Naruto universe or from some other dimension, which then leads to new adventures.
One popular plotline is Naruto fully embracing the power of the Nine Tails in a way that doesn't make him lose his humanity. For example, he could use the Nine Tails' chakra to perform new and powerful jutsu while still maintaining his bonds with his friends. Another is Naruto having to deal with the psychological impact of the Nine Tails' possession more deeply. Maybe he has nightmares or visions related to the Nine Tails' past. And a third could be a situation where Naruto's Nine Tails - related powers are somehow transferred to another character, creating a whole new set of challenges and relationships.
Well, a common plotline might be Naruto exploring the ancient secrets of the Nine Tails' power. This could involve him going on a journey to find out more about the origin of the Nine Tails and how it came to be sealed within him. Another plotline could be that Naruto's Nine Tails possession causes some sort of rift among the ninja villages. Some might see him as a threat, while others as an opportunity. Also, there could be a plot where Naruto has to train his mind to better control the Nine Tails' influence, and during this training, he discovers new abilities within himself.