After second watch last night Im more in love with my prediction that
@ Scoffed at.
Arya is going to kill the mountain. No Clegane bowl.
Allow me to make my case.
The Mountain is on Arya's list. The hound is fully aware of Arya's "list" from all the traveling they did together when she was his "prisoner". In that process the Hound obviously grew a fondness for Arya.
The show went out of the way to highlight the Hounds fondness for Arya in the last episode when the Hound meets with Brienne on the way to the Dragon pit. When Brienne says that "people need to be protected from Arya" (not exact quote, too lazy to scrub the episode but that was the gist) as opposed to Brienne protecting Arya, the hound got a big smile on his face. In the following scene we see the Hound walk up to the Mountain and say " you know whos coming for you" .
I think the Hound does not want to interfere with Arya getting the satisfaction of completing her "list".
The interesting thing here is will Arya kill the hound too? The Hound was on Aryas list. Is he still? I think it is kind of ambiguous. She crossed him off when she thought she left him for dead. I dont actually think he would even care if Arya killed him.