At first, Naruto might see the Nine - Tails as just a source of power, something that makes him strong. But as he grows, he starts to see it as a part of himself, an entity with its own feelings and history.
In a fanfiction where Tails meets Sonic for the first time, Tails might be in awe. Sonic's speed and heroic reputation could make Tails instantly idolize him. Maybe Sonic saves Tails from a dangerous situation, like a group of badniks. Tails, being the smart fox he is, could then offer to help Sonic with his gadgets, starting a great friendship.
Their interaction would likely be positive. Sonic, being the friendly hero, would probably greet Tails with a big smile. Tails would be shy at first but soon open up due to Sonic's warm attitude.
One possible reason could be some kind of strange chakra infusion. Maybe he comes into contact with a powerful and mysterious chakra source that mutates his body in a way that causes tails to grow. Another reason might be related to some ancient curse or prophecy. It could also be the result of a new jutsu he attempts that goes wrong and has unforeseen physical consequences.
One interesting 'tails female transformation fanfic' could be about a character who discovers a magical item that gradually transforms them into a female with a tail. This transformation might bring new powers and a different perspective on the world they live in.
One common theme is redemption. The characters often go back in time to correct their past mistakes related to the ten - tails, like preventing its misuse or saving lives affected by its power. Another theme is power exploration. Since the ten - tails is such a powerful entity, the fanfictions usually explore how the characters can better understand and control its power during the time - travel journey.
Another possibility is that the time - traveling Ten - Tails Jinchuuriki ends up in a time during a great war. Their presence could either end the war quickly if they side with one of the parties, or make it even more chaotic as all sides would either try to recruit or eliminate this powerful being. They might also have the power to change the outcome of certain battles and thus reshape the world's power balance.
In certain fanfiction, Naruto is indeed the Ten - Tails. This could be a really cool concept as it combines Naruto's already powerful nature with the ultimate power of the Ten - Tails. It might lead to stories where Naruto has to struggle with the dark side of such power, or he could use it to bring about a new era of peace in the ninja world. Maybe he would have to face off against old enemies who are now even more afraid of his new - found strength.
When they meet for the first time in fanfiction, Sonic could be running from Dr. Eggman and accidentally runs into Tails' hiding place. Tails is scared at first, but Sonic quickly reassures him. They start talking, and Sonic is amazed by Tails' intelligence and his ability to build things. Tails is inspired by Sonic's courage and they become friends right away, sharing their goals of stopping Dr. Eggman.
One possible plot is that Sonic is exploring a new area. He comes across a small workshop where Tails is building his first plane. Sonic is intrigued by the little fox and his inventions. Tails is initially startled by Sonic's sudden appearance but then gets excited to meet his idol. They start chatting about their adventures, with Sonic sharing his tales of running fast and Tails talking about his love for inventing things.