Personally, I'd love to see The Hound vs The Mountain too.
I know they are in different locations, but I can't help thinking that the writers must have kept him (The Hound) alive for some other purpose than simply going off to fight in the North against the white walkers. I think his character may seem a little lost being part of a large battle like that.
He's got personal scores to settle, whether that's with Brienne, Arya or his brother. It would be interesting to see how he'd react to meeting Arya again, since she didn't technically kill him, just left him for dead. I'm not sure if that's something you could get over, with time..
I'm probably wrong about that, but the unpredictability is one of the coolest things about this show. I didn't see Margaery's demise coming - I honestly thought there was another sub-plot developing there.