Her unique personality. Kushina was known to be feisty and strong - willed in the original Naruto series. In fanfiction, this can be amplified. For example, she can be the driving force behind a lot of changes in the village's policies or the way the ninja operate.
Often in fanfiction, there could be a story about Kushina's influence on Naruto's growth. It could show how her spirit or some hidden knowledge she left for Naruto helps him in difficult situations. It might also involve Kushina interacting with other characters like the other Jinchuriki in a different way than in the canon story, perhaps forming a sort of Jinchuriki alliance.
Another aspect could be that the fanfiction delves into how Kushina's presence affects Naruto's relationships with others. For instance, his relationship with Sasuke could be different. With a strong maternal figure in his life, Naruto might be more stable emotionally and be able to approach Sasuke in a more understanding way, which could potentially change the whole dynamic of their rivalry and friendship.
One possible fanfiction could be about Kushina finding Naruto in a situation where he's lost in a forest. Maybe she uses her unique chakra sensing abilities to track him down. It could be a heartwarming story of a mother reuniting with her son.
Minato could be a common character. Since he was Kushina's husband in the original story, his presence might add some interesting dynamics to the harem scenario. Another could be Jiraiya. His connection to Naruto and his reputation as a bit of a ladies' man could make for an interesting addition to the story.
One possible reason could be for Naruto's protection. Maybe she thought by banishing him from a certain place or situation, she could keep him away from a hidden danger that she knew about. For example, there might be a powerful enemy targeting Naruto specifically, and she believed that by making him leave, he would be harder to find.
Perhaps Naruto finds a way to travel back in time and prevent Kushina's death in the first place. He could use his Sage Mode or some other special ability to manipulate time. And then, by changing the past, he effectively'revives' her in the present. In the Naruto universe, there are all kinds of crazy powers, so time - travel isn't completely out of the question.
In some fanfictions, Kushina was sealed into Naruto through a complex jutsu. It might be a modified version of the sealing jutsu used to seal the Nine - Tails. The process could involve special incantations and chakra manipulations.
One common plot is Kushina finding Naruto in a different time or dimension. For example, she might end up in the future where Naruto has grown up and she gets to see the kind of person he has become. It's an emotional and interesting concept as she gets to witness all his achievements and the challenges he has overcome.
At first, they would likely be a bit awkward since they've never really interacted before. But soon, the bond of mother - son would strengthen. Naruto would share his adventures with Kushina, and Kushina would listen intently, offering her own experiences as an Uzumaki. They would grow to understand and support each other deeply, with Kushina guiding Naruto in both his personal and ninja life.