1) AU-NonMagical: Harry inherits a large family business. Snape was senior VP of another company, but was recently let go. He needs a job and Harry hires him.
2) Post-War: Harry saved Snape that night at the shack. And with his magic, helped Snape recover from the injury. He fell in love with Snape during this process, and decided that being with this man is his top priority in life.
3) AU-Magical: Harry and Snape are both Potion Masters and have admired each other's work for years. They decided to meet up in a potion conference.