Definitely Hinata. Her love for Naruto is well - known, and the awakening of the Sharingan could intensify her feelings and her presence around Naruto. Sakura too, as she has been an important figure in Naruto's life. And there could be some new female characters introduced, perhaps from a far - off clan that has heard about Naruto's new power and they send their most beautiful and talented female ninja to be part of his 'harem' to form an alliance.
Sakura would be a likely candidate. She has always had a connection with Naruto, and his new Sharingan ability might make her see him in a more romantic light. Hinata is also very likely, given her long - standing feelings for Naruto. In addition, Tenten could potentially be part of it, as she might be intrigued by Naruto's new power.
Sakura, Hinata, and Ino are the most obvious choices. Sakura has a long history with Naruto, and his new Sharingan could change their relationship dynamics. Hinata's deep love for Naruto would make her a central figure. Ino, being Sakura's rival in some ways and also a strong female ninja, might be interested in Naruto's new power. There could also be some minor female characters that play supporting roles in the 'harem', like the Kunoichi trainees who look up to Naruto and are attracted to his new, powerful image with the Sharingan.
In this fanfiction, perhaps the awakening of the Sharingan gives Naruto an edge in battles that makes the female ninjas admire him even more. The plot might involve Naruto being initially confused by all the attention from the girls, but then gradually coming to terms with it. For example, Sakura could see him in a new light and Hinata might be even more smitten. And this new dynamic could lead to various adventures and challenges as Naruto tries to balance his relationships with his duties as a ninja.
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.
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.
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.
One possible plot twist could be that Naruto's Sharingan has unique abilities not seen in the regular Uchiha Sharingan. Maybe it can combine with his Sage Mode in unexpected ways, like seeing chakra flows more clearly even in Sage Mode, enhancing his combat skills.
One possible triggering event could be the loss of a dear friend. Just like Sasuke's awakening of the Mangekyou Sharingan was related to the trauma of Itachi's actions. In a fanfiction, Naruto might witness the death of someone like Kakashi, who has been a mentor figure. This intense emotional pain and the sense of loss could be the catalyst for his Mangekyou Sharingan to awaken. It would add a new layer of depth to his character, as he would have to deal with this new power and the grief that came with its awakening.
Perhaps in the fanfiction, Naruto discovers a hidden connection between his Uzumaki heritage and the Sharingan. This newfound knowledge, combined with a great battle against a powerful villain, causes the Mangekyou Sharingan to awaken. It could also involve some sort of ancient prophecy or power that was long forgotten, and Naruto is the key to unlocking it which leads to the awakening of his Mangekyou Sharingan.
His relationship with Sasuke would become more complex. Sasuke, as an Uchiha, might feel threatened or intrigued by Naruto having the Sharingan. They could have a new level of rivalry or a new form of cooperation.
Other characters like Sakura might be involved as well. She could be curious about how Naruto got the Sharingan and how it changes his abilities. Characters from the Uchiha clan, even if they are just mentioned in flashbacks, could be important. For example, Itachi's actions and the secrets he held about the Sharingan could be explored more deeply in relation to Naruto now having this power. And then there are the villains. They might see Naruto with the Sharingan as a new and greater threat or as an opportunity to gain power for themselves.
It could be Sasuke. Since he has the Sharingan, in fanfiction about the Eternal Sharingan, he might be a central figure, evolving his powers further.