Fourth year. After an incident, Harry is assigned a mentor, not a bad idea with a tri-Wizard tournament to survive, but awkward if that mentor is Severus Snape. Not in the Slytherin, Snape and Dudley world.
Due to Harry performing accidental magic, Dudley becomes a wizard and attends Hogwarts in Harry's second year. How does Harry cope? Snape mentors Harry fic. No slash. CP in later chapters.
Continuing the Slytherin, Snape and Dudley story, Harry and Dudley enter Year three of Hogwarts. What happens when there's a mass murderer, a werewolf and everyone's least favourite DADA professor thrown into the mix?