Game of Thrones season 8, episode 4 all but confirmed Cleganebowl will happen in the next two episodes. Not only did one brother restate his intentions, the other signed his own death warrant by killing the nicest person on the show.
Despite winning a victory for humanity at the Battle of Winterfell, the reality of the wars still to come dampened the celebratory mood in the days after. Dany realized what an insurmountable hurdle Jon's parentage was, especially after watching how everyone credited him with heroism while largely ignoring her part in their win. Given her experiences with tyrannical queens, Sansa couldn't get over her mistrust of the Daenerys despite the aid Dany had generously given them. And while prostitutes and merriment surrounded him, Sandor Clegane remained focused on a singular goal that's dogged him almost his entire life. He growls to Sansa that "There's only one thing that'll make [him] happy," and when Arya asks why he's going to King's Landing, he mentions he has "unfinished business."
The Hound's pointed statements to Sansa and Arya and his determined journey south imply his intent is find and kill his brother, which means we're most likely going to see Cleganebowl before the show is through. Sandor made a promise to Gregor the last time they met in Game of Thrones season 7's finale, and with the dead finally brought to heel and Dany's armies heading to King's Landing, there's nothing else standing in the way of his mission to rid the world of his older brother. While the Mountain's enjoyed a somewhat sanitized image in recent seasons acting as Cersei's champion when she was at her most vulnerable, his execution of Missandei firmly planted him back in the ruthless villain category where he started.
At this point, it would be a bigger surprise if the confrontation between the Hound and the Mountain didn't happen. Not only has the show directly referenced the brother's rivalry on several occasions, the two shared one of Entertainment Weekly's promotional covers before season 8 kicked off. All signs point to Cleganebowl, but there's still a question of how Sandor will be able to get close enough to his brother to even attempt the match-up, much less finish it.
Sandor Clegane is a known (and highly recognizable) traitor to the Crown, so it's not like he'll be able to just walk up to the gates of King's Landing and knock. It's possible he'll just commit to fighting for Jon and Dany so he'll have access to his brother if they take the Red Keep, but considering what we just saw this week, that's a much bigger "if" now than it was last season. Jon and Dany are leading decimated armies, with almost no navy and a dragon who's far from the super weapon he once was. Given Sandor's famous lack of f*cks when it comes to the ruling classes, it's doubtful he'd risk a chance at eliminating his brother to aid in a regime change he likely doesn't care about. But there's more than one way to get inside the city, and Sandor's riding with someone who knows that fact very well.
Arya Stark spent nearly all of Game of Thrones season 1 exploring the Red Keep and knows at least one way to get inside the castle that would avoid direct confrontation. She discovers a way out through the sewers while exploring the dungeons in episode 5, "The Wolf and the Lion." That said, she might not even need to go that far - we haven't seen Arya exercise her tricks as a Faceless (wo)Man since the beginning of season 7. It's possible she and the Hound could aid each other on their separate missions and take out the head of the snake and her champion in dual combat. One can only hope she doesn't leave him to die this time...
Game of Thrones continues on Sundays at 9pm on HBO.