Naruto would be overjoyed to see his parents. Emotionally, it would give him a great boost. He would finally have the family he always dreamed of having around him.
Perhaps they return to impart some final wisdom to Naruto. Minato could teach Naruto some advanced sealing techniques that he hadn't mastered yet, and Kushina could share more about the Uzumaki clan's secrets and how Naruto can further harness his Uzumaki heritage.
Minato's character could be developed by showing his growth from a young, talented ninja to a more mature and responsible leader. His relationship with Kushina can play a big part in this. For example, through his love for her, he becomes more empathetic. Kushina, on the other hand, could be developed by exploring her past and how it affects her actions. Her time with Minato helps her become more confident and less impulsive.
There's a fanfic called 'Minato's Return'. In this story, Minato somehow returns and gets to see Naruto's growth. It's really interesting to see how he interacts with Kushina, who is also there in the story, and how they both guide Naruto in new ways. The descriptions of their family bond are heartwarming.
One interesting storyline could be Minato and Kushina's first meeting. Maybe it was a chance encounter in the Hidden Leaf village. Minato, with his natural charm, caught Kushina's attention right away. Another could be their training together, where they learn new jutsu and grow closer as they support each other through tough training sessions.
One great fanfic is 'The Early Days of Naruto' which beautifully depicts Minato and Kushina's relationship with baby Naruto. It shows their tender moments together and how they start to teach Naruto about the ninja world from a very young age. Another good one is 'Naruto's Family Beginnings' where the focus is more on the family dynamic. You can really feel the love Minato and Kushina have for their baby. And then there's 'Baby Naruto's World' which goes into detail about how Naruto's presence affects the village even as a baby, and how Minato and Kushina are at the center of protecting and nurturing him.
In some fanfics, Naruto might be made to hate Minato and Kushina due to a sense of abandonment. If the story is written in a way that emphasizes how Naruto grew up alone and suffering, he could project his anger onto his parents for not being there for him. For example, he might think that if they were alive, he wouldn't have had to endure the taunts and loneliness in the village.
In a fanfic, Kushina might be depicted as a source of great wisdom for Naruto. Since she was his mother, she could pass on some Uzumaki family secrets or just some motherly advice. Minato could be the guiding force in Naruto's development as a ninja. His skills and experiences could be something that Naruto looks up to. Naruto himself should be a complex character, with his own insecurities and doubts at times, but always pushing forward because of his parents' influence.
On a more personal level, they could face internal guilt. As Naruto grows up and achieves great things on his own despite their neglect, Minato and Kushina may realize what they've missed out on. This realization could cause them a great deal of internal turmoil and regret for not being there for their son when he needed them.