One drawback could be the strain on his body. The Sharingan is a powerful dojutsu, and Naruto's body, which is used to his own chakra system, might not be able to handle it well at first. This could lead to fatigue and over - exertion during battles.
He could change in terms of his relationships too. His friends might be both in awe and a bit wary of this new power he has. Naruto himself might struggle with the newfound power, as it's not his natural ability. He might become more introverted as he tries to master it privately, not wanting to accidentally harm his friends with this new, potentially dangerous power.
One potential plotline could be Naruto using his new powers to finally bring true peace to the ninja world. He could use the Sharingan to understand the root causes of conflicts by seeing through people's true intentions and the Rinnegan to reshape the world in a more harmonious way.
Yes, there are likely some Naruto fanfictions out there where Naruto has a blue Sharingan. Fanfiction writers often come up with creative and unique ideas like this. They might explore how Naruto's character would be different with this new ability, or how it would change the existing storylines in the Naruto universe.
If Naruto had the Sharingan in fanfiction, it would likely lead to some interesting power - up scenarios. He could combine the power of the Sharingan with his own massive chakra reserves. This might make him an even more formidable opponent. His training would also take a new turn as he'd have to learn to control and master the Sharingan's abilities, like the different levels of the Sharingan and their unique powers.
In fanfiction, Naruto could get the Sharingan through a blood - related connection that is revealed later. For example, it could be discovered that he has some distant Uchiha ancestry that gets activated somehow. Maybe a powerful enemy's attack awakens this dormant Uchiha blood in him, and as a result, he develops the Sharingan. It could also be that a powerful sage or a new character uses their own unique powers to bestow the Sharingan upon Naruto for a specific purpose.
A new character could be introduced as a Sharingan master. For example, a wandering ninja who has been secretly studying the Sharingan for years. He might have no allegiance to any village and travels around, looking for worthy pupils to inherit his knowledge. He could have unique techniques related to the Sharingan that no one has seen before.
Well, it could have the ability to time - travel. Imagine Naruto using it to go back in time to prevent some of the great tragedies in the Naruto world, like the Uchiha massacre. This would change the whole storyline in a very interesting way.
One potential implication is that Naruto's fighting style would change drastically. He would have to learn how to use the Sharingan's abilities like predicting movements and copying jutsus. This could make him an even more formidable opponent.
The character with the Mangekyou Sharingan might lose their eyesight due to overuse in the new and unfamiliar world of the crossover, but then gain a new form of power that is related to the crossover world. Maybe they lose their Sharingan sight but gain the ability to see magic auras or something similar. This would be a huge twist for both the character and the readers.
One potential implication is that Naruto would gain enhanced visual abilities. The Sharingan can copy jutsu and see through illusions, so Naruto would be even more powerful in combat. For example, he could quickly learn new enemy techniques during battles.