There's also a chance that a Hogwarts teacher discovers Harry. Dumbledore, for example, seems to know a great deal about the Room of Requirements. He might have a feeling that Harry is there and decide to check on him, either to offer guidance or to make sure Harry isn't getting into any dangerous situations.
It could be a curious house - elf. They often pop up in unexpected places in Hogwarts. The house - elf might be on some errand and accidentally stumble upon Harry in the room.
Perhaps a Slytherin student who has been secretly following Harry. In fanfiction, there are often rivalries and intrigues. This Slytherin might suspect that Harry is up to something in the Room of Requirements and wants to find out what it is to gain an advantage over him.
Well, he might discover some long - lost magical artifacts. Since the Room of Requirements provides what one needs, there could be something really powerful that helps him in his future battles against Voldemort or other dark forces.
His friends like Ron and Hermione might help him. They are always there for Harry and if they believe that Harry has a valid reason for hiding from Dumbledore, they would likely do whatever they can to keep him hidden and safe. For instance, Hermione could use her intelligence to create spells or distractions to keep Dumbledore off Harry's trail.
There's a chance he'll run into Ron. Maybe Ron is looking for some new wizarding sweets at Honeydukes, and they end up sharing some of the treats while they continue their shopping spree. And it's possible that they'll see Neville there too, who is searching for some rare plant seeds for his Herbology studies.
He might meet a mysterious old wizard who has been traveling the seas for centuries, gathering knowledge of different magics. This wizard could be a great source of wisdom for Harry.
He could meet Hagrid. Hagrid often goes to Diagon Alley, so it's possible he'll run into Harry while he's shopping and give him some useful advice about what to buy or not to buy.
He could meet centaurs. Centaurs are known to live in the Forbidden Forest in the Harry Potter universe, and if Harry is living in the woods, it's very likely he'd cross paths with them. They might teach him about astronomy and prophecy in a more in - depth way than he learned at Hogwarts.
He could find Itachi's collection of jutsu scrolls. These scrolls might contain rare and powerful jutsus that Naruto has never seen before, which could give him new ideas for his own training.
His friends Ron and Hermione would likely be the first to find him. They are always by his side in most situations. They know Harry well and would notice if he was in trouble.
Maybe some of the younger ninja like Boruto or Sarada could stumble upon Naruto's cave while on a training mission. They could be curious about this strange hermit - like figure in the cave and later realize it's Naruto. This could lead to a mentoring - like relationship between Naruto and the new generation.
He might meet a mysterious old wizard. The wizard could be the one who brought Ash to the room, either for a purpose or by accident. The wizard could then guide Ash on a journey, or perhaps be an antagonist who has some dark plan for Ash.