Well, in a fanfiction, Naruto might inherit the Mangekyou Sharingan from an unexpected source. Maybe it's a gift from a dying Uchiha who sees Naruto's potential to bring peace. With the Mangekyou Sharingan, Naruto could enter a new dimension of power. He might start to have visions related to the Uchiha's past traumas and secrets. This could drive him to find a way to heal the rift between the Uchiha and the rest of the ninja world. He would have to balance his old values and fighting styles with the new and dangerous power of the Mangekyou Sharingan.
A possible plot is that Naruto gets the Mangekyou Sharingan in a fight against a powerful enemy. With this new ability, he is able to see the enemy's weaknesses more clearly. He could use the unique abilities of the Mangekyou Sharingan, like Tsukuyomi or Amaterasu, in his own unique way, since he is not an Uchiha. For example, he might combine it with his Rasengan. This would lead to new and exciting battles and also a journey of self - discovery as he tries to understand this alien power within him.
One idea could be exploring a different origin for the Mangekyou Sharingan. For example, what if it was related to an ancient Uchiha pact with a hidden spirit? Another could be a story where a non - Uchiha character somehow gains the ability to use the Mangekyou Sharingan and has to deal with the consequences. Or perhaps a fanfiction where the Mangekyou Sharingan has additional powers that are not shown in the original Naruto series, like the ability to time - travel.
One idea could be a story where the protagonist discovers a new and unique power within the Mangekyou Sharingan that has been lost for generations. This power could give them an edge in battles against powerful foes like the Akatsuki. Another plot could revolve around a character who has to overcome the dark side effects of using the Mangekyou Sharingan, such as losing their sight at a faster rate. They could search for a way to counter this while still using the power to protect their village.
A possible plot could be that Dark Naruto's Eternal Mangekyou Sharingan has a special power that can merge the minds of people. He uses this to merge with the good Naruto inside him. Through this merge, he battles his inner demons and slowly transforms back to his normal self while also gaining the combined strength of both his dark and light sides.
One implication is a change in his power level. The Mangekyou Sharingan gives access to powerful jutsus, so Naruto would be even more formidable. He might also face new enemies who are interested in his new ability.
Another plot could be that the Sharingan is given to Naruto as a last - ditch effort to save the village. A dying Uchiha gives Naruto his Sharingan, and Naruto has to figure out how to incorporate it into his already powerful set of skills. He might have to deal with the jealousy or suspicion of some of his comrades who are wary of the Sharingan's power.
In the fanfic, Naruto having the Eternal Mangekyou Sharingan could lead to a change in his relationships. His friends might be both in awe and a bit worried about this new power he has. Sasuke, being an Uchiha, could have a complex reaction. Maybe they would train together to better understand and control this power. Naruto could also use it to try and bridge the gap between the Uchiha and the rest of the village even more, as he now has a part of the Uchiha heritage in a very powerful way.
One possible plot could be that Naruto gains the Eternal Mangekyou Sharingan through some sort of unexpected and mysterious event. Maybe it's a result of a powerful jutsu gone wrong that fuses his own chakra system with that of a Sharingan - user. This new power could initially be difficult for Naruto to control, leading to him having to isolate himself to figure it out. As he masters it, he could use it in unique ways, like combining the Sharingan's abilities with his own Rasengan techniques, creating entirely new and powerful attacks.
Another idea is a crossover with a superhero universe. Imagine a character like Sasuke with his Mangekyou Sharingan powers suddenly in the Marvel or DC universe. He could team up with heroes like Spider - Man or Batman. His Sharingan's ability to copy moves could be really interesting when facing off against super - villains.
Another plot idea is that Naruto gets a Sharingan in a parallel universe. In this universe, he has to learn how to control it quickly while also dealing with the different political situations. Since the Sharingan is so powerful, many might try to either use Naruto or take the Sharingan from him, creating a lot of action - packed and suspenseful situations.
One popular plot idea could be a character with Mangekyou Sharingan traveling back in time to prevent a major tragedy in the Naruto universe. For example, stopping the Uchiha massacre. This would create a whole new set of events and relationships. Another idea might be a time - traveling ninja using the power of the Mangekyou Sharingan to gain knowledge from the past and use it to change the future of their clan. They could learn ancient jutsu or secrets that were lost over time.