I felt that it was too long, too intricate. I think it would have helped having a shorter narrative of how our protagonist came upon the mirror. It was certainly fascinating, but because the fic starts off with him already having found the mirror (and feeling its effects), the very detailed back story only delays the reader finding out what happens, what he sees. It is definitely a necessary point, but there isn't really a draw for how he found the Mirror because he already has.
Read the story now
Other than that, I absolutely loved this! Very creative, very unique. I thought it was great that you did not have any of Rowling's characters, Maruader-era, HP-era, or second generation. I think it added to the story, in the sense of Hogwarts becomes its own functioning institution rather than about Harry. I do not personally like your idea of adding a fifth house, but it worked well with your story, especially with his musings at the end about them possibly adding a sixth house. I did not see that ending coming AT ALL! It was fantastic - gave me chills!