One possible secret could be about Naruto's real parents' untold past. Maybe there are hidden details about how his father and mother met that were not in the original story.
It could be Naruto himself. As he grows and faces new challenges, he might stumble upon these secrets, either by accident or through his own determination to learn more about his world and his place in it.
It could be revelations about Naruto's family heritage that was not fully explored in the original series. Maybe secrets about his parents' past deeds or hidden powers within his bloodline.
One possible plotline could be Naruto's discovery of his heritage in a different way than in the original story. Maybe he stumbles upon some hidden clues about his father being the Fourth Hokage earlier. Another could be his friendship with Sasuke starting much earlier, like they had some secret adventures as kids that shaped their future rivalry.
Well, it could be that Naruto has a secret about his parents that's darker than we thought. Maybe he found out some negative things about his father's past actions that he's been hiding. For example, perhaps his father had to make some really tough and morally grey decisions during the war that Naruto now has to come to terms with.
One interesting 'Naruto has heritage revealed' fanfiction could be where Naruto discovers a hidden clan connection. For example, he might find out he has ties to an ancient and powerful Uchiha - like sub - clan that was long forgotten. This could lead to new jutsu development and a whole new understanding of his place in the ninja world.
You can start by looking on fanfiction platforms like FanFiction.net or Archive of Our Own (AO3). These sites have a large collection of Naruto fanfictions, and you can use their search functions. Just type in 'Naruto has heritage revealed' and browse through the results.
In this 'naruto childhood revealed fanfiction', we could be exposed to Naruto's feelings towards the villagers who shunned him as a child. It may show his internal struggle between wanting to prove himself to them and hating them for their treatment. There could also be more about his pranks and how they were not just for fun but also a way for him to get attention, as well as his relationships with other outcast kids in the village if there were any.
Well, in some fanfictions, it could be during a life - or - death battle. Maybe when he's facing an extremely powerful enemy that he can't defeat with his known abilities. His friends could be in great danger, and that intense situation triggers the revelation of his hidden power.