The Akatsuki could be antagonists. They might see Kushina's return as a threat to their plans. Since Kushina is a powerful Uzumaki, her presence could disrupt their schemes related to the tailed beasts. They could try to stop her from reaching Naruto or even try to capture her for their own purposes.
In such a fanfic, perhaps Kushina manages to break free from the afterlife's constraints. She could use some sort of ancient jutsu that allows her to return to the living world. Then, she starts her journey to find Naruto. Maybe she disguises herself as an ordinary traveler at first to observe Naruto from afar, seeing how he has grown and all the hardships he has faced. Once she reveals herself, Naruto is initially shocked but then filled with joy at the thought of having his mother back.
Kushina could also seek the help of other powerful shinobi. For instance, she might ask Minato for assistance. Since Minato was Naruto's father and also a very skilled ninja, together they could come up with a plan to reunite with Naruto. Maybe they would use some sort of space - time ninjutsu to get closer to him.
Naruto's reaction could be complex. On one hand, he would be extremely excited. But on the other hand, he would be worried about the consequences. He has learned about the balance of the ninja world and might be afraid that bringing Kushina back could have negative impacts. However, his love for his mother would probably overpower those concerns in the end, and he would still strive to make her return a reality.
Some members of the ANBU themselves could be antagonists. They might think that Naruto belongs with them and that Kushina is interfering in their affairs. They could try to keep Naruto in the ANBU by any means necessary, including using force or setting up traps for Kushina when she tries to approach Naruto.
Another aspect could be the emotional impact on Naruto. Having his mother back would be a huge boost to his mental state. He's always longed for family, and Kushina's return would fill that void. It could change his perspective on life and make him even more determined to protect the village and his loved ones.
In a fanfiction, Kushina could want Naruto to master a unique jutsu that she had always dreamed of but was unable to achieve herself. This could be a jutsu that combines the power of the Uzumaki clan's sealing techniques with some other element, like wind. And she would be guiding Naruto in his journey to master it, through some kind of spiritual connection.
The plot could be that Kushina wants Naruto to reunite with his father. She could use her spiritual connection to Naruto to plant the idea in his mind. Naruto then embarks on a journey to find out more about his father, Minato. Along the way, he discovers secrets about the past, including how his parents really met and fell in love. This could also lead to him unlocking new powers related to his family lineage.
One possible plot development could be that Kushina's spirit somehow manages to break through the boundaries between the afterlife and the living world. She then tries to communicate with Naruto, perhaps through dreams at first. This could lead to Naruto starting to search for a way to bring her back permanently, involving him in some sort of ancient and dangerous ritual that he discovers in some long - forgotten scroll.
One possible plot development could be that Kushina uses her influence within the village to start a formal request for Naruto to be returned from the ANBU. She might gather support from other characters who also care about Naruto's well - being, like Iruka or Jiraiya. This could lead to a series of political discussions within the village leadership about the role of the ANBU and whether Naruto's skills are better utilized elsewhere.