Obviously, Harry Potter is the main character as it's about him hiding his power. His friends, like Hermione Granger and Ron Weasley, are also likely to be main characters. They might be in the dark about his hidden power at first, which could create some interesting dynamics in their relationships.
Well, one possible plot could be that Harry hides his power to protect his friends from dark wizards who might target them if they knew how powerful he really was. Another could be that he hides his power to secretly investigate a mystery within Hogwarts without drawing attention.
One idea could be that Naruto hides his power to protect his loved ones from being targeted. For example, if his enemies knew his true strength, they might go after Hinata or Iruka. Another could be that he's hiding it as part of a long - term strategy to overthrow a greater evil that is secretly observing the ninja world. Maybe he wants to lull them into a false sense of security before making his big move.
It could be a new character. Maybe a powerful ninja from a far - off land who has heard rumors of Naruto's strength and is determined to draw it out and defeat him to prove his own superiority. This antagonist might be secretly spying on Naruto and trying to find ways to force him to show his true power.
One example could be a fanfic where Naruto pretends to be a weak genin even during the Chunin Exams. He might let others underestimate him while secretly observing his opponents' moves and strategies. This allows him to surprise everyone when he finally reveals his true power during a crucial battle.