There are several potential plotlines. For example, the Saiyan race could be on the verge of extinction due to some threat, and the ability of their tails to grow back becomes a glimmer of hope for their survival. The Saiyans then have to figure out how to fully utilize this new - found ability. Another plotline could be that a group of Saiyans with regrown tails forms a special unit. They face discrimination from the traditional Saiyans who view them as abnormal at first, but they prove their worth through their unique abilities related to the regrown tails.
Well, a possible plotline might involve a young Saiyan who was born without a tail but then suddenly starts to grow one. This causes confusion and excitement in the Saiyan community. Or perhaps there is a prophecy related to Saiyans with regrown tails that sets off a series of adventures to fulfill it. Also, there could be a plotline where the regrown tails have some unexpected powers that the Saiyans have to learn to control.
One plotline could be that a Saiyan discovers a way to accelerate the tail - growing - back process and uses it to gain an advantage in a tournament. Another could be that an evil force targets Saiyans' newly regrown tails for some dark purpose.
The tail - growing - back process might have a profound impact on Saiyan culture. In their society, the tail has always been a part of their physical identity. If it grows back, it could change the way they view their own strength. For example, they might develop new fighting styles that incorporate the use of the tail more effectively. And it could also influence their mating rituals as the tail is often associated with Saiyan's primal instincts.
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.
Maybe Sakura gets the Nine - Tails' power accidentally. This could lead to her struggling to control it at first. As she tries to master it, she could form new relationships with other characters who either help or hinder her progress. For example, some might be jealous of her new power, while others might be intrigued and want to study it.
One possible plotline could be that Tails is killed by a powerful enemy during a battle to protect his friends. Maybe a new, extremely strong villain emerges and Tails sacrifices himself to buy Sonic and the others time to escape. Another could be that there is some sort of accident in Tails' workshop while he is working on a dangerous experiment, leading to his untimely death. It could also be that in a dark alternate universe, Tails is targeted and assassinated by an evil organization that wants to disrupt the balance of power in the world he lives in.
In a 'Tails Turns into a Robot' fanfiction, it could be that an ancient technology is accidentally activated by Tails. This technology fuses with him and turns him into a robot. He then discovers that he has a special connection to an ancient robotic civilization. He has to navigate his new identity, learn about this civilization, and decide whether to embrace his robotic self or try to find a way back to his normal form all while facing challenges from those who fear or want to use his new - found powers.
It could lead to all kinds of interesting plot twists. Maybe they come back stronger or with new abilities.
One possible plotline could be that Naruto uses his newfound ability to go back in time to prevent some of the tragedies that occurred in the past related to the Bijuu. For example, he might try to stop the initial capture and sealing of the Bijuu into different vessels, which could change the entire history of the ninja world. Another could be that the Bijuu themselves, with Naruto's help, go back to a time when they were free and try to establish a new world order where they are not feared and hunted.
Another plotline could involve their relationships. Sonic, as the younger one, might be more naive in some ways and look up to Tails for guidance. Tails, in Sonic's body, could find it strange to be the center of attention and the one expected to lead. Their friendship dynamic would change completely. Maybe Sonic would start to envy Tails' natural intelligence more, while Tails would realize how much pressure Sonic was under all the time.
One possible plotline could be about two characters who initially dislike each other. They are brought together in a situation where absinthe is involved, perhaps at a party or a seedy bar. As they consume absinthe, their inhibitions are lowered, and they start to see each other in a different light. They begin to share their secrets and fears, and gradually, their relationship deepens.
Perhaps it was the result of a special training or a new form of energy absorption. Gohan might have been exposed to a new type of ki or energy that stimulated the regrowth of his tail. For example, if he was in a place with a high concentration of a certain type of energy that interacted with his Saiyan cells in a way that caused his tail to grow back.