A Dragon Is not a Slave
Dany says this twice. Once to the creepy guy in Astapor who sells her the Unsullied, and then again to Jon when they're in the dragon pits outside Kings' Landing. And others say similar things. Jorah tells Dany that a dragon cannot be tamed, even by its mother, and Tyrion implies as such when he unchains the dragons in Mereen.
So what, if anything, might this mean for Viserion? If a dragon is not a slave and cannot be tamed, could that mean something for future battles? Is Viserion completely bound to the Night King's will, or might there still be part of him that belongs to Dany?