He might be shocked. In the fanfic, if he sees things like his friends' future achievements or the new threats that emerge, it would surely startle him. For example, if he discovers that one of his friends has a family of their own and has become a great leader in the village, he would be proud but also a bit surprised at how much has changed.
With his usual determination. Naruto has always been one to face difficulties head - on. So, even if he's shocked by what he sees in the fanfic's future, he'll likely try to find solutions immediately.
His father, Minato, could be a likely candidate in a really dark fanfiction. Maybe he was resurrected in an imperfect way and his mind was warped, causing him to turn on Naruto.
Perhaps his family consists of a group of fellow banished ninjas who become his surrogate family. There could be a strong - willed female ninja who becomes his partner, and together they have a child. And also an old sage - like figure who acts as the family's wise elder, guiding them through the difficulties of living in exile.
It could also be due to some family curse or prophecy. In the Naruto universe, there are often strange powers and curses. Maybe there was a prophecy that said Naruto's family had to keep their distance from him to protect him from some greater evil. So, they chose to neglect him in order to keep him safe, even if it was a difficult choice.
Another popular choice for Naruto's girlfriend in fanfiction could be Hinata. In the official story, they end up together. Fanfiction can expand on their family life, perhaps delving into how Hinata's family and Naruto's new - found Uzumaki family interact. For example, stories could be written about family traditions they combine or how they deal with the pressures of being a Hokage's family.
In some fanfiction, Naruto might seek revenge on his family if he finds out they have a dark secret that goes against his values. For instance, if he discovers that his family was involved in some unethical or cruel actions in the past. However, this would be a huge departure from the Naruto we know in the original series, as he is usually very forgiving. But in the realm of fanfiction, anything can be explored. Also, it could be a situation where Naruto believes his family has been hiding their true identities from him, and these identities are associated with something he despises.
His father, Minato, could be one. Maybe there's some time - travel or alternate reality situation where Minato has been corrupted in some way and Naruto has to fight him to set things right.
One possible reason could be that they are too focused on their own problems or duties. Maybe they are involved in some important shinobi business that takes up all their time and attention, causing them to overlook Naruto.