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.
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.
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.
His power level would skyrocket. Having all nine tailed beasts means he has an incredible amount of chakra at his disposal. He could potentially use their combined powers in various ways, like creating massive and powerful jutsu. For example, he might be able to combine their unique abilities to create a super - offensive or defensive move that no one could counter easily.
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.
He could develop the ability to transform into a more powerful form that combines the features of all nine tailed beasts. Maybe he could have nine tails of different colors or shapes, each representing a different tailed beast, and with each tail comes a special power.
In this fanfiction, Naruto would gain the ability to manipulate the elements in new ways. Each tailed beast is associated with a different element or combination of elements. By having all of them, he could create complex elemental attacks that are not possible before. For instance, he could combine the fire of one tailed beast with the water of another to create steam - based attacks. His healing factor might also be enhanced as some tailed beasts possess regenerative abilities. This would make him even more difficult to defeat in combat.
In such fanfiction, Naruto may gain the ability to heal himself more quickly. Since the tailed beasts possess a great amount of life - force energy, Naruto could tap into that to accelerate his own healing process. His mental abilities could also be enhanced. He would be able to withstand greater mental pressure and use the tailed beasts' collective wisdom to make better decisions in battles. Moreover, he could potentially create a chakra shield that is extremely strong, using the combined chakra of all the tailed beasts to protect himself and his allies.
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.