Rarepair

Rare pairs might also slash together people or creatures that are considered unnatural, such as a human character with a magical creature such as a centaur or a house elf, or Dumbledore with any young student from Hogwarts.

If a Yuletide reader requested a rare pair (Silver Surfer/Aunt May?) in The Marvel Cinematic Universe Avengers, that story will now assemble with its more popular Hulk/Iron Man and Captain America/Bucky Barnes fanfic colleagues in the Avengers tag.

With rarepairs and crackships, it's the unlikeliness or even impossibility of their coming together that gets you going.