It could be Hinata. Even though in the original story they are openly married, in a fanfic scenario, there could be a situation where their relationship was kept secret for some time and she gave birth to a son secretly. Maybe due to some threat to the village or Naruto's safety.
Well, one plot twist could be that the son has some unique power that neither Naruto nor his friends are aware of at first. Maybe it's a power that was dormant until a certain event triggered it.
Well, in many fanfics, the authors might create scenarios where Sasuke shows his secret feelings for Naruto. It could be through small gestures, like protecting Naruto when no one is looking or having internal monologues about his deep connection to Naruto. However, in the original manga and anime, their relationship is complex and while there are strong bonds of friendship and rivalry, it's not as explicitly 'liking' in a romantic sense as some fanfics might depict.
I'm not sure specifically what 'naruto secret fanfic' is about as there could be many different ones. It might be a fan - created story that contains some hidden or less - known aspects of the Naruto universe, like a new character's backstory or a different take on a well - known plot.
Maybe Sasuke. Although they are rivals, they also have a deep bond. Sasuke might have come across a unique jutsu during his travels and decide to teach it to Naruto so that they can both become stronger and better protect their village.
You can start by searching on fanfic websites. There are many platforms dedicated to Naruto fanfics where you might be able to find it if it's publicly available.
A 'naruto secret rinnegan fanfic' might center around the idea of a hidden Rinnegan technique. In this fanfic, the author could be exploring how a character obtains the Rinnegan in a very different and secretive way compared to what we've seen in the official Naruto story. It could also look at how the Rinnegan interacts with other unique abilities in the Naruto universe, perhaps creating new and unexpected combinations of powers.
Well, in a 'the secret technique naruto fanfic', it could be that a minor character, say Rock Lee, uncovers a secret technique that compensates for his inability to use ninjutsu. This new technique could be a unique form of taijutsu that gives him an edge over his opponents. It might also explore how this discovery affects his relationships with other characters in the Naruto universe, like how his friends and rivals react to his newfound power.