In some fanfictions, when Harry Potter goes through the mirror, he might find himself in a parallel magical world. This world could have different versions of his friends and enemies, and he may have to face new challenges to find his way back to his own world.
In this fanfiction, perhaps a new character is an old wizard who has been trapped inside the mirror for centuries. He has a lot of ancient magic knowledge and he becomes a mentor - like figure to Harry. He helps Harry understand the true power of the mirror and how it can be used to save the wizarding world from a new threat.
One interesting plot twist could be that the mirror shows not just desires but also hidden threats to the wizarding world. For example, it might reveal a new dark magic that Voldemort is planning to use, which was previously unknown. Another could be that when Harry looks into the mirror this time, instead of seeing his parents, he sees a future version of himself that has turned to the dark side, which would completely change his perception of himself and his future.
He might gain some extraordinary magical abilities. For example, he could be able to control time or have a deeper understanding of magic spells that were previously difficult for him.
One possible plot could be that Harry Potter enters the RWBY universe through the veil. He might bring his magic skills and knowledge with him. He could then interact with the RWBY characters, perhaps teaching them some new spells or learning about their unique abilities. His presence could also disrupt the balance in the RWBY world, leading to new conflicts and alliances.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
In Harry Potter fanfiction, the mirror can often represent self - discovery. For example, the Mirror of Erised shows a person's deepest desires. Fanfic authors might use it to explore a character's hidden wants and fears more deeply than in the original books. It could be a tool for a character to face their true selves.
Some RWBY characters might be in awe. Ruby, with her love for all things cool and powerful, would probably be very excited to meet Harry and learn about his magic. Weiss, on the other hand, might be a bit more reserved and initially skeptical. Blake could be curious about where he came from and what his world is like. Yang might see him as a potential ally in fights against Grimm and be eager to test his strength.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.