She might be overjoyed. After all, Naruto is her son. She could immediately rush to find him and embrace him tightly, tears of happiness streaming down her face.
One consequence could be a shift in the power dynamics. If Kushina is alive and now reunited with Naruto, they could form a powerful duo. This might make their enemies more cautious or even lead to new alliances being formed against them. Also, Naruto's morale would be greatly boosted, and he might become even more determined in his goals, which could speed up the overall plot development.
Well, one possibility is that Dark Naruto would be overjoyed. After all, Kushina is his mother. But at the same time, he could be very conflicted. His dark side might make him suspicious of her motives. He could test her in various ways to see if she's really who she says she is. And Kushina, on her part, would probably be heartbroken to see what her son has become.
One possible plot twist could be that Kushina's survival was part of a grand plan by an unknown entity. Maybe she was hidden away to protect Naruto from some greater threat that was yet to be revealed. Another aspect could be the emotional turmoil Naruto would go through. He'd be overjoyed, of course, but also confused and maybe a bit angry that he was made to believe she was dead all this time.
If Naruto found Kushina alive in a fanfiction, it might lessen his focus on Sasuke for a while. He'd be more centered on getting to know his mother. But in the long run, it could also make him stronger and more determined in his rivalry with Sasuke. With Kushina's support, Naruto would have a new source of motivation to prove himself not just to Sasuke, but also to his mother.
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.
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.
You can try websites like Fanfiction.net or Archive of Our Own (AO3). On Fanfiction.net, you can simply search for 'Kushina finds Naruto' in the search bar and it will show you a list of relevant fanfictions. AO3 also has a large collection, and their tagging system can help you narrow down your search to find exactly what you're looking for.
One thing that could happen is that Dark Naruto would be filled with a mix of emotions. He could be angry at the village for hiding this from him, yet also feel a glimmer of hope. This discovery might make him question his loyalty to the forces that led him to become 'dark'. He might also feel a sense of longing for the family he thought he had lost.
Well, one possibility is that Naruto would be overjoyed. He has always longed for his mother's love. He might spend days just talking to her, sharing all his experiences, from his training with Jiraiya to his battles against the Akatsuki. Kushina would probably be extremely proud of him and would start teaching him some Uzumaki clan jutsus that were passed down in the family. And Naruto's relationship with the village would also change. He would no longer be the orphan that everyone pitied. Instead, he would be the son of a hero, which could bring a whole new dynamic to his standing in Konoha.