My vision of Clegane Bowl:
The Mountain will be revealed to be a wight (Benjen Stark like) - might happen in a fight with Grey Worm dealing supposed to be deadly blows that the Mountain shrugs off (and ultimately kills the attacker). Hound to win the day, but how? Well why, kill it with fire , of course! It's the foundation of their whole story - the Hound overcoming his fear for fire to defeat his tormentor.