One day, a canon character of your choice discovers an old portrait that causes a strong reaction in them. Before they're aware of it, they've been transported back in time, to meet the person in the portrait ... (again, feel free to go any way with this but would very much like it if it was Historical)
A tale of Tom Riddle at Hogwarts. Like Harry Potter, Riddle never went home for Christmas. Instead, he spent his holidays looking for secrets hidden in the castle. What he found would cause death and sorrow for years to come.