As we know, she will demand a trial by combat, as is her right by the laws of Westeros. We've seen several of those trials by combat so far:
- Bronn (for Tyrion) vs. the Captain of the Guards of the Eyrie (for Lysa Arryn)
- Beric Dondarrion vs. Sandor Clegane
- Oberyn Martell (for Tyrion) vs. Gregor Clegane (for Cersei Lannister)
But who will be his opponent, chosen by the Faith? Many have speculated that it might be his brother, Sandor "The Hound" Clegane, whose hatred for the Mountain goes very deep. The assumption that Ian McShane as Septon Meribold / the Elder Brother might bring the Hound back only fueled the hype for a CleganeBowl further:
But now another very interesting possibility has been brought to conscience by Rhiealla, a poster on "Winter is coming", after having seen this scene in an after episode 4 video:
This is Loras Tyrell, wearing rags and a more pious hairstyle, obviously after being released from prison by the High Sparrow. As we've seen in episode 4, the time in the cells under harsh conditions (and maybe even torture) has kind of broken the spirit of this sweet summer child. As actor Finn Jones says: "He's definitively a changed man. [...] He just wants to do anything he can to get out of that cell."
Taking all this in account, it could very well be Sir Loras, known as one of the best knights in Westeros, who's gonna buy his way into freedom and / or prove his new-found faith by fighting as the champion of the faith in the upcoming trial by combat. Loras is right there, ready to be picked up and used by the High Septon; whereas the Hound, even IF he was brought back as a changed and religious man, would have to perform quite a voyage to return to King's Landing, and his motivation for doing so seems totally unclear.
By the way: This would be a beautiful reminiscence to season 1, where Loras beat the Mountain at the tournament (though only at jousting, and probably not by playing completely fair) and afterwards almost got killed by him. The man who saved Loras' life at that occasion was - the Hound. Which opens up yet another alternative: IF CleganeBowl really happens, the Mountain might be defeating his brother Sandor; then in the last moment Loras would intervene and save the Hound, thus repaying the service.
After all speculation, it is still worth remembering that there does not have to be a trial by combat: the option can be denied by the judges, and this might very well happen in this case.