Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
In the manga, Sasuke doesn't regain his Rinnegan. The story progresses without this particular change. The power he had before is maintained without reverting or regaining additional abilities related to the Rinnegan.
Well, in some versions of this fanfiction, it might be that Naruto uses a powerful and forbidden jutsu to defeat an overwhelming foe. This jutsu drains his life force, and Sasuke, who has just realized how much Naruto means to him, catches Naruto as he collapses. It could also be that Naruto is poisoned, and Sasuke tries his best to save him but fails in the end, and Naruto passes away in his arms.
Naruto could use his own growth and strength as an example. He shows Sasuke that he has become stronger not just in power but also in his understanding of the ninja world. He tells Sasuke that together they can change Konoha for the better, that Konoha has changed since he left and is now a place where he can be accepted and contribute. He might also share his own experiences of loneliness and how having Sasuke back would complete him in a way.
She might start by using her knowledge of Naruto's past. Since she is his mother, she knows things about his early life that no one else does. She could use these memories to create an emotional connection with him. Maybe she'll show up in his dreams first, like a hazy memory, and then gradually reveal herself in the real world.
They could be trapped in a genjutsu world where they have to rely on each other to break free. In this world, their past differences don't matter as much, and they have to work together. Naruto uses his determination, Sakura her knowledge, and Sasuke his skills. Through this shared experience, they start to understand and get along with each other.
One challenge Naruto might face is Sasuke's own will. Sasuke may be so consumed by his thirst for revenge or his dark path that he doesn't want to be brought back. Naruto has to break through that mental barrier. Another challenge could be the external forces that are influencing Sasuke, like the people or organizations that have turned him to the dark side. Naruto has to deal with them as well.
The setting also matters. If the fanfic is set during a time of war, Naruto's decision to hold back Sasuke might have different implications compared to a peacetime setting. In a war - time fanfic, it could be related to the greater good of the village or the ninja alliance. And the way Naruto holds back can vary, from not using his full strength in a fight to using words to stop Sasuke from making a wrong move.
Redemption is also a big theme. Sasuke is often seen as a lost soul who needs to be redeemed. Naruto's efforts are not just about bringing him back physically but also about saving his soul. For example, in many fanfictions, Sasuke has done some bad things, and Naruto is there to make him see the right path again.
In many Narusaku fanfics, Naruto might bring Sasuke back through the power of their friendship. Naruto's unwavering belief in Sasuke could be a key factor. He may constantly remind Sasuke of their past together, the bonds they formed as teammates in Team 7. For example, Naruto could recall the times they trained together, their shared dreams, and how they protected each other in battles. This could slowly chip away at Sasuke's resolve to stay away and eventually lead him to come back.