Actually, Harry did make it into the most secure room in the Dept. of Mysteries at the end of his Fifth Year.
Another dimensional cross-over Harry? A Dark Grey Harry and a very innocent one combine when Voldemort tries to bend a Light Ceremony to his will. Hopefully not cliché-ridden.
When would be the best point in time for an over-powered Harry to return to his past? Right before the First Task!
Harry escapes the summer before his fifth year, and comes back with a different attitude.