I am having trouble coming up with a fate terrible enough for that monster. Here are some ideas that might suffice.
  1. Ghost rapes Ramsay to death
    In revenge for the rape of Sansa of course
  2. Put him in between Dany's dragons and let nature take its course
  3. Cut out both of his eyes and let him and Arya fight to the death
  4. Put a white blonde wig on him and let him get abducted by a Dothraki khalasar
  5. Have him spend a long weekend with Jorah Mormont on a river cruise up Old Valyria
  6. Tie him down below the moon door of the Eyrie and throw his hounds at him as a sick game of pin the tail on the donkey
  7. Hang him on a cross from The Wall to slowly freeze/starve to death
  8. The Stark direwolves tear him limb from limb but he's still alive enough to be roasted by Dany's dragons
  9. He has to go on an eternal road trip listening to Robin Arryn's whining
  10. Brienne shoves Oathkeeper up his ass
  11. Theon's sister castrates Ramsay, cooks his penis, dips it in Dijon mustard, and feeds it to him until he chokes to death on it