Yes, Sarada awakens Mangekyou Sharingan. The manga builds up to this moment with a series of intense battles and character developments that lead to this power awakening.
One interesting concept could be how Kakashi uses the eternal Mangekyou Sharingan's unique powers in new and unexpected ways. For example, he might be able to create more complex illusions or have enhanced teleportation abilities.
The fanfiction might show Kakashi growing in terms of his understanding of the Sharingan's true nature. As he delves deeper into the abilities of the eternal Mangekyou Sharingan, he could gain a broader perspective on the Uchiha and their powers, which in turn enriches his own character development. He may start to see things from an Uchiha - like point of view, which was previously foreign to him. Also, the new power might attract new enemies or allies, and through these interactions, Kakashi will have to adapt and grow. For example, if new enemies target him because of his powerful Sharingan, he'll need to find ways to outwit them and protect those he cares about. This will surely contribute to his character growth.
Kakashi's character would likely become more confident in his abilities. With the Dual Mangekyou Sharingan, he has access to greater power, which could boost his self - assurance. He might take on more dangerous missions without hesitation.
Perhaps a storyline where Kakashi's dual Mangekyou Sharingan makes him a target. Enemies from other villages might see his new power as a threat and try to either steal his eyes or kill him. This could lead to a series of intense battles and chase scenes. Kakashi would then have to rely on his wits and the help of his friends and comrades to stay alive. In the process, he could form new alliances and strengthen old ones, all while protecting his newfound power.
In many Naruto fanfictions, Itachi is often depicted as an extremely powerful Mangekyou Sharingan user. His original abilities in the canon, like Amaterasu, are further enhanced in fan works. Sasuke is also a common focus. His Mangekyou Sharingan's powers can vary greatly from one fanfiction to another, sometimes with new and unique jutsu created by the authors. Another character could be Madara. Fanfiction writers love to explore the full potential of his Mangekyou Sharingan, often giving him abilities that were only hinted at in the original series.
One creative plot twist could be a character who was thought to be a normal ninja suddenly reveals they have a Mangekyou Sharingan with a unique power that changes the whole power dynamic in the story. For example, a side character who has been helping the main character all along, but then uses their Mangekyou Sharingan to stop an impending doom that no one else could. Another could be that the Mangekyou Sharingan has the ability to swap the powers of other ninjas, creating chaos and new alliances.
His personality could become extremely cold and calculating. The power of the Mangekyou Sharingan might make him see others as mere pawns. He no longer values the bonds he once had in Naruto's normal state. Instead, he is only focused on achieving his own dark goals, whether it's power, revenge, or domination.
For his relationship with Kakashi, it could be complex. Kakashi, having his own experiences with the Sharingan, would be in a unique position to guide Naruto. But at the same time, he might be worried that Naruto's new power could attract unwanted attention. Naruto's bond with the villagers could also be affected. Some might see him as more of an Uchiha than a Uzumaki now, and there could be those who fear his power. His relationship with Jiraiya, if he were still alive in the fanfic, would also change. Jiraiya would need to teach Naruto how to use this power for good and not let it corrupt him.
With Sasuke, their relationship would become more complex. Sasuke might be both jealous and intrigued by Naruto having the Mangekyou Sharingan. Jealous because it was always an Uchiha thing, and intrigued because he wonders how Naruto will use it. This could lead to more intense rivalries and also more opportunities for them to understand each other better.