Kakashi can be quite popular too. His wisdom and experience could play a role in guiding Naruto as he deals with the power of the Rinnegan. Also, fans might be interested in seeing how Kakashi reacts to Naruto's new - found power and how it might affect their bond.
Pain is often a very popular character in Rinnegan fanfiction. His original use of the Rinnegan in the series made him quite an interesting character, and in fanfiction, people like to explore his backstory further or give him new powers. Nagato, who was the original owner of Pain's body, is also popular. He has a complex history and his connection to the Rinnegan can be explored in many ways.
Sasuke Uchiha is often a popular choice. Given his natural Uchiha abilities and his connection to the Rinnegan in the original Naruto story, he's a likely candidate in fanfiction. His brooding nature and quest for power make him a great character to explore further with the added power of the Eternal Rinnegan.
Well, Obito Uchiha can also be considered an iconic character in Rinnegan - related fanfictions. His connection to the Sharingan and his later acquisition of the Rinnegan through various means is a rich area for exploration. And, there could be an ancient ancestor of the Rinnegan Clan, a character only mentioned in the fanfiction, but whose legend looms large over the clan and its members.
In Rinnegan Naruto Clan Wars fanfiction, a character like Nagato could be really interesting. He already had the Rinnegan and his story could be further explored in the clan wars setting. Maybe he could be a key figure in a new clan or an outcast trying to regain his power. Another interesting character could be a new OC (Original Character) created specifically for this fanfiction. This OC could be a young ninja who discovers they have the Rinnegan and has to navigate the dangerous world of clan wars. And Kakashi could also be an interesting character. His Sharingan experience might give him some unique insights when dealing with the Rinnegan - wielding opponents in the clan wars.
There could be an original character created in the fanfiction who has a unique form of the Rinnegan. Maybe they have combined it with another kekkei genkai, making them a powerful force. Such a character could have abilities that are unheard of and thus be one of the most powerful.
Sasuke is often a common character. Given his connection to the Uchiha clan and his own powerful ocular abilities, it's easy to imagine him being involved in a story about sharing the Rinnegan. He could be a key figure in either using the shared Rinnegan for good or being tempted by its power.
Naruto himself is often included as he has a central role in the Naruto universe. Also, characters like Hinata, Sakura might be part of the harem. And Sasuke could also be involved in some way, either as a rival or an ally to the main character with the Godlike Rinnegan.
One popular storyline often involves Hinata somehow obtaining the Tenseigan and how it affects her relationship with Naruto, especially considering Naruto has the Rinnegan. Maybe they team up in a new and powerful way against an old foe.
One of the most compelling new characters could be a hybrid ninja who has both Sharingan and Rinnegan abilities from birth. This character would have to struggle with the immense power and the expectations that come with it. Another great character could be a scientist - type who studies the Sharingan and Rinnegan, trying to unlock their full potential. They might not be a traditional fighter but their knowledge and inventions add a new dimension to the fanfiction. There could also be a character who was once an ordinary ninja but was experimented on and now has a strange combination of Sharingan and Rinnegan - like powers.
The Akatsuki as a whole could be antagonists. In fanfictions, the Akatsuki might be interested in the power of Naruto's Evil Rinnegan. They could try to capture him to extract the power or use him in some way for their own goals. Pain, being a powerful member of the Akatsuki, could also be a specific antagonist. His own ideology about pain and peace might clash with Naruto's new, potentially more chaotic state with the Evil Rinnegan. Additionally, new original antagonists created specifically for the fanfiction could be common. These could be dark, powerful figures who are drawn to the power of the Evil Rinnegan and want to either possess it or destroy Naruto to prevent its misuse.