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.
If Naruto gets all the tailed beasts in the fanfiction, it would change the power dynamics completely. He could use their unique abilities in combination. For example, the Shukaku's sand control along with the Nine - Tails' chakra. This would make him a force to be reckoned with not only in combat but also in terms of politics and diplomacy in the ninja world. Other villages might see him as a threat or as a new leader to follow. And his relationships with his friends and allies would also evolve, as they would have to adjust to this new, super - powerful Naruto.
In the 'naruto gets all the tailed beasts fanfic', Naruto's life would change completely. He would no longer be just an ordinary ninja with a lot of potential. Having all the tailed beasts within him means he has access to an unimaginable amount of power. He could potentially unlock new forms or abilities that are a combination of his own and those of the tailed beasts. This could make him a force to be reckoned with not only in his own village but across all the ninja villages. His relationships with his friends might also change. They might look at him with a new sense of awe and respect. And Naruto would have to learn to control this new - found power, which could be a huge challenge in itself. But if he manages it, he could reshape the ninja world in ways no one has ever imagined.
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.
The relationships can be developed through shared battles. When Naruto and the fem tailed beasts fight side by side against common enemies, they can start to trust each other more. For instance, if they face a powerful rogue ninja together, they'll learn about each other's combat styles and strengths.
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.
Kurama is often a very popular character. As the Nine - Tails, he has a long history with the main Naruto storyline and his complex relationship with Naruto makes him interesting in fanfiction. He has a lot of power and a fiery personality.