Well, she might face the challenge of Naruto's trust. Since she has been absent for so long, Naruto may not trust her immediately. Also, the ninja world is full of threats. There could be enemies who try to stop her from getting close to Naruto. And Naruto has his own sense of identity which is deeply rooted in being an orphan and a self - made ninja. Kushina has to respect that and find a way to fit into his life without disrupting it completely.
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.
Maybe they enlist the help of other powerful ninjas. For instance, they could ask Tsunade for medical support in case Naruto is injured. They could also get intel from Shikamaru, who is known for his intelligence. By gathering a team of reliable and strong ninjas, they increase their chances of getting Naruto back successfully.
One consequence could be that it messes with the natural balance of the ninja world. Maybe the cycle of life and death is disrupted, and this causes some strange phenomena to occur, like abnormal weather or chakra imbalances.
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.
One challenge could be the powerful ninja from Kumo. They are known for their strength and combat skills. Naruto would have to face them in order to rescue Kushina.
Trust could be a big issue. Since they've broken up before, there might be lingering doubts. Alec might worry that Magnus will leave again, and Magnus might be afraid that Alec will let the rules of the Shadowhunters come between them. They need to work through these insecurities if they want to get back together.
In certain fanfic ideas, Naruto could have made a deal with some sort of otherworldly being. He might have sacrificed something of great value, like a part of his own chakra or a future ability, in exchange for Kushina's life. Another possibility is that he finds a way to reverse the very nature of death by using some unique properties of the tailed beasts' chakra that he has access to.
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 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.
One challenge could be adapting to the team's dynamics. Team 8 has its own way of operating, and Naruto is used to his own style with his friends. For instance, they might be more methodical in their planning, while Naruto is more of a 'jump - in - and - figure - it - out' type. So, he'd have to learn to follow their lead sometimes.