History AU with decade/era of your choice
Ideas:
Victorian Era London: A is a chimney sweep who loves to read, B is the bookkeeper's child
USA in the Roaring 20's: A is a jazz musician performing at a bar, B catches A's eye while on the bandstand
Age of Piracy in Spain: A is a famous pirate, B is the badass pirate that takes over A's ship.
The American Frontier: A has lived in the small town all of their life, when they meet B (who is just passing through), A decides to leave with B