Madara Uchiha could be an interesting main antagonist in this crossover. Since he already has strong ties to the Sharingan and was a powerful force in the Naruto series. If he somehow got his hands on the Rinne Sharingan's full power, he would be even more formidable. He could use it to manipulate reality or time, making it extremely difficult for Naruto and his friends to stop him.
Kaguya Otsutsuki could also serve as the main antagonist. Imagine if she regained her strength and also acquired the abilities of the Rinne Sharingan. She could use her already vast powers along with the new ones from the Rinne Sharingan to try and take over the world again. Her connection to the origin of chakra and the Sharingan would make her a very complex and powerful enemy in this crossover scenario.
It could be a rogue ninja who has also obtained a corrupted form of the Rinne Sharingan. This antagonist would use the power for his own gain, causing chaos in the Naruto world. He might be seeking to overthrow all the existing shinobi villages and establish his own rule, using the unique abilities of the Rinne Sharingan like the ability to create powerful illusions that can affect entire villages at once.
It could be Orochimaru. Since he has always been interested in obtaining powerful abilities, he might see Naruto with the Sharingan as a threat and try to take it for himself.
It could be Orochimaru. He has always been interested in powerful abilities like the Sharingan. In this fanfiction, he might try to steal Naruto's newly acquired Sharingan for his own experiments and to increase his power even more.
It could be a rogue ninja from a forgotten clan who has a vendetta against the Hidden Leaf. This ninja might be seeking to obtain the power of the Mangekyou Sharingan for himself, and sees Naruto as a threat. So, he tries to stop Naruto from fully awakening it and uses all kinds of dark jutsu to attack Naruto and his friends.
Well, in the 'Naruto awakens the Rinne Sharingan fanfiction', it's possible that Naruto's character arc takes a whole new turn. With the Rinne Sharingan, he might be able to see the chakra flow in a much more detailed and different way. Maybe he could even communicate with the tailed beasts on a deeper level, using the unique abilities of the Rinne Sharingan. This could lead to new forms of cooperation between them and Naruto, which would be really exciting for the story.
It could be an evil ninja who also has a powerful form of the Sharingan. He might be jealous of Naruto's new ultimate Sharingan and wants to take it for himself. This antagonist could use his Sharingan powers to create illusions that are so powerful, even Naruto has a hard time breaking free from them.
Well, it depends on the specific fanfiction. There are countless Naruto fanfictions out there. Some authors might decide to give Naruto the Rinne Sharingan to make him even more powerful or to introduce new plot twists. For example, it could be a result of some special training or a mysterious power transfer in the fictional world of the fanfic.
Naruto himself is often a common character. Since it's related to the Naruto universe, he might be a part of the story, either as a friend or a rival to the character with the Rinne Sharingan. Sakura could also be a common character, perhaps as one of the members of the harem.
Sasuke is often a popular character in such fanfictions. His natural Sharingan abilities make it interesting to explore how he would interact with the Rinnegan powers. Fans like to write about him achieving new levels of power by combining or evolving his Sharingan in relation to the Rinnegan.
Madara Uchiha is another popular character in Naruto Rinne Sharingan fanfiction. Since he was one of the first characters to be associated with the Rinnegan (a precursor to the Rinne Sharingan in some ways), fans are interested in seeing how his character would be even more powerful or how his motives might change if he had the Rinne Sharingan. There are often stories that revolve around his further rise to power or his redemption with the help of this powerful eye ability.