Well Gregor is obviously Robert Strong, and the gravedigger is most likely the Hound. The idea is that Cersei will demand trial by combat with Zombie Mountain as her champion, and the Faith of the Seven will pick the Hound as their champion. How the Hound goes from gravedigging to the champion, no one knows, but it's not entirely unlikely. The guy who says "The Hound is dead. Sandor Clegane is at peace." is a religious man, and the Quiet Isle (where Brienne sees the gravedigger) is a refuge for those who follow the Faith of the Seven. I can't believe that Sandor has some deep religious awakening, but I can see him faking it to have a place to stay and possible revenge against his brother and Cersei. The main issue with this is that I believe Brienne notes that the gravedigger is injured or walks with a limp. Time to bust out those faith miracles.
Further evidence for Cleganebowl comes from the show. Like the book, Sandor has an ambiguous death. However, the key point is the family that Hound and Arya run across. The bald man with the little creepy quiet girl that serves them rabbit stew. The man is deeply religious, as we see a lot of iconography of the seven gods in his home and he also recounts a prayer. The Hound scoffs at his faith, which I believe is going to be serve as dramatic irony. The Hound is beat to shit in the show, how is he going to survive? I highly suspect he crawls back to that family as they are in the general vicinity, or perhaps members of the faith come across him on their way to King's Landing with the High Sparrow.
Cersei demanding a trial by combat is pretty much going to happen, so realistically, who are the Faith going to pick? If they pick some noname, it's not so exciting. The only twist I can think of is if the Faith pulls out the "Trial by Seven" from the Dunk and Egg stories where each side has seven combatants each.
