The big threat that’s loomed over Westeros since the beginning of the series has been the Night King and his undead army in the North. And now that he also has an undead dragon to do his bidding, will the Night King ride Viserion on Game of Thrones? Daenerys’ dragon could be the key ingredient they needed all along to get past The Wall and invade the rest of the continent.
The main thing that’s kept White Walkers out of the majority of Westeros on Game of Thrones has been the magic that binds them from passing through The Wall. The Children of the Forest explained this magic back in Season 6 through one of Bran’s visions that showed them turning one of the First Men into the first White Walker and the Night King himself. They created magic carvings in The Wall to keep the White Walkers out once they realized how much of a threat they were. But now that Viserion is a wight, or basically an ice dragon, he might be what the Night King needed all along to get his army of the dead past The Wall. Or, possibly, through it.
Fans have been waiting for ages to see an ice dragon, so Viserion becoming a wight isn’t going to be for nothing.
And since the Night King can control any wight or any White Walker he creates, then Viserion will no doubt be under his control too, making it possible to tame and ride him with the flick of a wrist. Because wights are harder to kill than mere humans, and dragons are harder to kill than a lot of other things, a wight dragon is going to be almost indestructible. The Night King could ride Viserion through The Wall, effectively crumbling it to the ground and bypassing the magic carvings the Children of the Forest made so long ago.
This would enable the rest of the dead to march over the mountains of snow and into Westeros. He could even use Viserion to get over The Wall. But in order for this to happen, the Night King will have to ride Viserion, and the way things are looking, that was likely his whole point.
In fact, he didn’t look at all shocked when Daenerys came riding in with all of her dragons to save the day North of The Wall. He calmly grabbed one of many ice spears and threw it with a scary amount of ease, almost as if he were trying out for the Olympics.
He may have been planning to steal one of Dany's dragons all along. And why would he take one if not to ride it during the next big battle?