The Hound is easily one of my favorite characters (Martin really seems to do a better job with the misfit types), so I hope there's more to his story.
I mean look at the guy, think there is a reason. I love his books, too and he is a very talented writer.
My hope is that when the red witch revives Jon he decides to abandon the NWs because ... well they just tried to kill him. He then goes overseas and joins Danny and becomes one of the dragon riders.
His vows were unto death and if he dies they are fulfilled.
He won't have to go overseas, the dragons will be in Westeros soon enough.
They will need someone with leadership ability too, something Daenerys definitely lacks. Perhaps Tyrion will counsel her, but where she has governed it always has not went well. She is no war leader as well. In many ways I think she is the ultimate red herring, all early signs point to her in the prophecy. But so far, he has not always been a hero, not led well. I get why people like her, Emilia Clarke is smoking hot. But I think it will take 3 as is suggested here. Drogon with her, and two other riders, Tyrion is a good general, he was at Blackwater at least. Jon, has did well leading men until betrayed.
One thing about Dany's troops is the Unsullied could not even beat up knife wielding guys in robes. They are low quality, even if she gets Dothraki, her dragons are her best asset. But something is buried in the Crypts of Winterfell, too, I think that may given Jon a dragon.