Another option could be an original character. This character could be a descendant of an ancient clan that has a special connection to the Ten Tails. They start off as an unknown, but as the story progresses, they become the key to the fate of the Ten Tails. This would allow for a fresh take on the Naruto universe and the Ten Tails story.
Sasuke could also be a great main character for this kind of fanfic. His power and his complex relationship with Naruto could add a lot of depth. Maybe he discovers a new way to combat the Ten Tails that Naruto doesn't know about, and they have to work together in a new way.
The Akatsuki could still be the main antagonists. They might be interested in Naruto's tails for their own dark purposes, like using their power to create a super - weapon or gain control over the ninja world. Another option could be a secret group of rogue ninjas who have a vendetta against the Uzumaki clan and see Naruto's tails as a symbol of their power that they want to destroy.
Perhaps an ancient evil spirit that was once sealed away by the nine - tails long ago. Now it senses the nine - tails' presence in Naruto and wants to take revenge on it by destroying Naruto. This antagonist could have powers that are a match for Naruto and the nine - tails combined.
Sasuke could be a main character in a Naruto 11 Tails Fanfiction. He has always been Naruto's rival and equal in many ways. His skills and knowledge of the ninja world could be crucial in dealing with the 11 - tails. Maybe he would have a different perspective on how to handle the situation compared to Naruto.
It could be a new ninja who has a special ability to sense the Zero Tails. This ninja might be an outcast in their village because of this strange ability, but it turns out to be the key to dealing with the Zero Tails.
Madara Uchiha could also be the main antagonist. Even if he was defeated before, in this fanfiction, he could come back with a new plan related to the Ten - Tails. He might want to use Naruto as a vessel to fully revive himself and then take over the Ten - Tails' power.
Obviously, Naruto himself would be the main character. Since it's about him growing tails, he's at the center of it all. The Nine - Tails would also play a crucial role. It's the source of his tail growth. And his friends like Sasuke and Sakura are likely to be important too. They'll probably be involved in Naruto's journey and how he deals with his new situation.
Obviously, Naruto is often a main character. Since it's about the Ten - Tails in the Naruto universe, he's usually at the center of the story, whether he's trying to defeat it, control it, or understand it.
Another idea is that the Ten Tails' power has some sort of connection to the Greek gods in the Percy Jackson world. Maybe it was once a creation of the gods or has some ancient link. Naruto, being the host of the Ten Tails, is drawn to the Percy Jackson world to uncover this mystery. Percy helps Naruto navigate this new world while also trying to figure out if this new arrival is going to cause chaos or bring a new form of balance.
Naruto himself could be the main character in a Shinju fanfic. He has always been about finding new ways to bring peace and understanding. His connection to the nine - tails could play an interesting role if he were to interact with the Shinju. Maybe he could try to convince the Shinju that the ninja world has changed and it doesn't need to be a source of fear and destruction.
Sasuke Uchiha could be a great choice as the main character. He already has a complex backstory and a lot of room for character development. His journey from seeking revenge to finding his own path in the Naruto series can be further explored in a fanfic.