It might be related to a connection with some powerful entity in the Wave arc. For example, if there's a secret shinobi from the past whose power is tied to the Rinnegan and Naruto somehow forms a bond with that power source, it could lead to the awakening. Also, the intense emotions and challenges he faces in the Wave arc might be a catalyst, pushing his chakra to a new level and awakening the Rinnegan.
It could be a water - based kekkei genkai. Since the Wave arc has a lot to do with water, it would make sense for Naruto to awaken a kekkei genkai related to water. This could give him an edge in battles, especially in the Wave region where water is abundant.
One implication is that he becomes much more powerful. The Rinnegan gives him access to powerful abilities like the Six Paths techniques, which would change the power dynamics in battles in the fanfiction. He could now face stronger enemies more easily.
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.
Naruto's relationships with his friends in Konoha would also shift. They might look up to him more, but at the same time, they could be a bit intimidated by this new power. For example, Shikamaru might be more cautious around Naruto as he tries to figure out how this new power affects Naruto's decision - making. And with his new abilities, Naruto might have to keep some secrets from his friends to protect them, which could strain their relationships a bit.
He might gain enhanced perception. This could allow him to see chakra flow more clearly, similar to how the Sharingan can. So in battles, he can anticipate his opponents' moves better.
Naruto's relationship with the elders of Konoha would also be affected. The elders might see him as more of a threat now, especially if they don't fully understand the nature of his Mangekyou Sharingan. They could try to keep him in line or even restrict his movements. However, Naruto, with his new power, might be more confident in standing up to them and demanding changes in the village's policies. This could lead to a power struggle within Konoha, with Naruto at the center, trying to balance his newfound power and his loyalty to the village.
Yes, in some fanfictions, Naruto can be depicted as having the Sharingan. Fanfiction writers often like to play with different character abilities and combinations, so it's not uncommon to see Naruto with the Sharingan in their creative works.
The Sharingan in such a fanfiction could be used as a tool for a Fairy Tail character to gain an edge in their battles. They might initially struggle to control it, but as they learn more about it, they become a formidable force. For instance, they could use the Sharingan's genjutsu abilities to deceive opponents in the Fairy Tail battles.