Interesting theory ive heard today is that the Nights King (Head of the white walkers) is actually the deceased Rhaegar Targaryen and is Jon Snows father from the Rape of Lyanna Stark. Which is why they had that 'look' when they come into close contact north of the wall. It may also explain the kings blood that Jon Snow is meant to have according to Mellisandre and why his sword didn't shatter when fighting the White Walkers. Which would link to 'a song of ice and fire'
Add the theory that its 3 dragons for 3 Targaryens that could be Jon, Danny and maybe Tyrion. Jon maybe able to Warg from the Stark side and thus able to tame the problematic Drogon