1: Harry specializes in tracking down illegal potion smugglers. When a dangerous new potion hits the streets, its effects sound familiar to Harry, who remembers its formula being scrawled in the margins of the Half-Blood Prince's book. But Snape is dead. Or is he?
2: Everyone expects Harry to join the Aurors, marry Ginny Weasley, and have babies. Instead, Harry decides to take a year off the see the world. Isn't it a coincidence that in every exotic locale he runs into the newly-acquitted Severus Snape?
3: Harry is an Unspeakable, whose research is the Veil. Without his superiors knowledge, he's been working to bring Sirius back. One day, however, instead of Sirius, Severus Snape emerges from the Veil.