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.
In fanfiction, Harry could meet some unexpected allies while hiding in the Room of Requirements. Maybe a group of students who have also found the room by chance and they form a secret society within it. They could share knowledge and spells that are not taught in the regular Hogwarts curriculum.
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.
Well, it might be a secret room where the projector shows prophecies. Harry, Ron and Hermione could stumble upon it while exploring the castle. The projector could be enchanted to only show relevant prophecies when the right person enters. For example, it might show a prophecy about the next big threat to the wizarding world that only Harry can see and understand.
In fanfiction, Harry staying at Malfoy Manor could lead to some unexpected alliances. He could end up befriending a house - elf there who helps him in his stay. This house - elf might know secret passages in the manor that Harry can use to his advantage. Or, Harry could even find a way to turn Draco Malfoy to his side during his stay, which would be a huge shift in the power dynamics considering Draco's family's loyalty to Voldemort.
Well, he could encounter some dark artifacts that have a strange pull on him. Maybe he stumbles upon a cursed object that starts to influence his thoughts in a negative way. Since Knockturn Alley is full of dodgy characters, he might also be recognized by some Death Eaters in hiding, which would put him in great danger.
His friends from Camp Half - Blood could be helping him. Annabeth, with her intelligence, might be devising plans to keep Percy hidden and safe. Grover could use his skills as a satyr to find safe hiding places and gather resources for Percy.
Well, if Harry obliviates everyone, it's a huge deal. Imagine all the memories gone! Regarding hiding fanfiction, it could be that he wants to prevent some controversial or dangerous ideas in the fanfiction from influencing people. But obliviating everyone is not ethical. Maybe he could have found other ways to deal with the fanfiction situation, like talking to the authors or setting some rules.
They could discover a hidden part of the Room of Requirement that no one else has found. It could be filled with ancient magic that only they can access. This discovery could lead to new adventures and challenges for them.
The showers could start shooting out different potions instead of water. For example, a truth - telling potion that makes everyone spill their secrets. Or, the shower heads could transform into small magical creatures that sing while you bathe. Also, the floor of the shower room might become a portal to a different part of Hogwarts or even a magical realm.
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.
It could be full of mystery and suspense. Maybe there's a hidden treasure or a secret passage.