Perhaps an ancient evil that was sealed away long ago has been awakened by the presence of all the tailed beasts in one person. This ancient evil could have a vendetta against the tailed beasts and see the reader as a means to destroy them once and for all. It could be a powerful demon or a dark ninja from a forgotten era, with powers and abilities that are a mystery even to the most knowledgeable shinobi in the current Naruto world.
It could be a new, powerful group that has emerged with the sole purpose of taking the tailed beasts from the reader. They might have some unique jutsu or technology that makes them a formidable foe. Maybe they are a group of rogue ninja from different villages who banded together out of envy for the reader's power.
Well, in such a fanfiction, the reader having all the tailed beasts could lead to a really complex plot. First, they would have to deal with the internal struggle of controlling such immense power. Maybe they start as an outcast in the ninja world because people fear their power. But as time goes on, they learn to use the tailed beasts' powers for good. They could form alliances with different villages, using the power of the tailed beasts as a deterrent against common enemies. And they might also discover secrets about the origin of the tailed beasts that no one else knew, which could change the whole perception of the ninja world.
In this fanfiction, Naruto could control all the tailed beasts because of his unique chakra nature. His chakra is very special and has a high compatibility with the tailed beasts' chakra. Also, his experiences with Kurama might have given him insights on how to handle other tailed beasts. He probably communicates with them on a spiritual level and convinces them to work together with him.
Well, in many fanfictions, Naruto might use his strong will and the power of his Uzumaki heritage. His natural chakra reserves are huge, which helps. He also has a deep understanding of friendship and the tailed beasts' pain from his own experiences. So, he can connect with them on an emotional level and gradually gain their trust, thus being able to control them.
It could be Orochimaru. He has always been interested in power and the secrets of the tailed beasts. He might see Naruto's transformation into a tailed beast as an opportunity to study and use that power for his own evil purposes. He could try to manipulate Naruto in his new state, perhaps by using genjutsu or other underhanded tactics.
One interesting storyline could be about the tailed beasts forming an alliance outside of the control of humans. They might work together to protect their own kind and the world in a different way than the shinobi. For example, they could create their own hidden land where all chakra - based creatures are welcome.
Perhaps an organization similar to Akatsuki. They might see the king of beasts as a powerful tool to achieve their goals, whether it's collecting all the tailed beasts or just ruling the world with an iron fist. They would send their strongest members to try and capture the king of beasts, and Naruto would have to stop them. In this fanfiction, this antagonist organization could have new and more powerful abilities compared to the original Akatsuki in the Naruto series.
In many fanfictions, the tailed beasts' personalities are shaped by their experiences. The Five - Tailed Horse is usually depicted as noble and proud. It has a certain elegance in the way it behaves. The Six - Tailed Slug is more nurturing and gentle, perhaps because of its slug - like nature. The Seven - Tailed Horned Beetle is often seen as mysterious and a bit aloof. It keeps to itself and doesn't easily open up. When it comes to the relationship with Naruto, these personalities play a big role. For example, Naruto might have an easier time getting along with the more friendly - natured tailed beasts at first, while having to work harder to understand and befriend the more reserved ones.
Sakura could be a great help. Her medical skills could come in handy if Naruto gets injured during his battles or difficult encounters while trying to free the tailed beasts. Also, her combat skills have improved over time, so she could fight alongside Naruto.
It could be a rogue ninja who wants to capture the ten - tailed wolf for its power. He might be someone who was once shunned by the ninja village and now seeks revenge by obtaining this ultimate power source.
It could be the Akatsuki. Since Naruto has all the tailed beasts, they would see him as a major threat to their plans of collecting the tailed beasts for their own purposes.