6. Sandor Clegane
This idea is great because it ties in with one of my favourite theories:
CleganeBowl. The idea is that Qyburn's creation, Ser Robert Strong, who was made in order to protect Cersei, is actually the reanimated corpse of Gregor Clegane, aka the Mountain (this is all but 100% confirmed at this point). He'll be fighting her side in a trial by combat, and on the side of the Faith will be the Hound. Yes, yes, he's supposed to be dead. But there is a theory that he's still alive, working as a gravedigger on the Quiet Isle (which would mean he has now turned to religion). Thus, the two Clegane brothers are going to face off in a fight to the death. Sandor is the younger Clegane brother, so could be the valonqar, and winning the trial would sentence Cersei to death. If you want to get really literal, it's very easy to imagine him choking the life out of her. GET HYPE!