I do think that there is a decent possibility that Sandor will eventually get revenge on his brother, but I don't see it happening in the trial by combat.
The main reason for this is 'plot armor'. If the mountain loses his trial by combat that means Cersei would die way too soon, even before Tommen dies. Not to mention how it would affect Qyburn's story line and Jamie's story line.
Tommen has to die before Cersei dies. Maybe the Sand Snakes will assassinate him.
Once Tommen dies, Margery will no longer have a reason to stay in King's Landing, so she will return to high garden and marry Littlefinger's gift (Gendry), and then Margery will be able to claim the iron throne again.
Of course after Tommen dies, as Jamie is no longer king's guard, I think that he is heir to the Iron Throne now due to some distant Baratheon relation, so Jamie might claim the iron throne.
This will also fit in better with the story line in the North. Once Petyr defeats the Boltons, he'll become Warden of the North and he'll join Sansa and liberate the riverlands. At some point it will be difficult for Petyr to remain in good relations with the Lannisters/Crown, but fortunately for Petyr Tommen will be dead by then so the Lannisters can't take away any of his titles. At which point Petyr will align the North/Riverlands/Vale with the Reach/Stormlands and support Margery/Gendry on the iron throne. As Gendry and Margery are both popular with the common people, they might be able to defeat the high sparrow and take back King's Landing. Petyr aligning the North with the Reach would also help fit in with Sam's story line.
Around this time, Daenerys will be landing in Westeros with her Dothraki/Unsullied/Dragon/Ironborne/Mercenary army, probably in Dorne, although maybe in the Reach/Stormlands. If Cersei is dead by now (maybe Jamie kills her before she tries to burn down king's landing with wild fire), as Jamie and Bronn still have good relations with Tyrion, the Lannisters will likely side with Daenerys. I think Dorne will side with Daenerys, but it may require regime change first. Not to mention Pentos and perhaps other Free Cities like Lys (due to Varys connection) and Volantis (after being invaded) will support Daenerys. Although Bravos and the Iron Bank will likely support the North due to their loan to Stannis and relationship with Petyr.
Things seem to be leading up to the creation of two massive factions: North/Wildlings/Riverlands/Vale/Reach/Stormlands/Bravos vs Westerlands/Dorne/Dothraki/Dragons/Unsullied/Freecities/Ironborne, which will engage in a massive war to control the iron throne. This will also give us Littlefinger vs Varys. Towards the end of the war the others will break down the wall and start heading south. Eventually the 2 sides will agree to peace due to the others and come together to defeat them.
As for Loras, I meant that he will fight for himself against the accusations that he is gay. He will fight against the 7.