A Night King centric episode
I think it would be cool to see a NK centric episode. One that would be a largely silent episode when focusing on him in the present but also seeing him in the past via Bran. More than anything, I only want to know two things: Is his conquering of Westeros simply based on revenge and why is he called the Night King (and how is it people know to call him that when they see him.)
Its obviously not the same as the legend of the 13th Nights Watch Commander from the books since in the show this guy was the original WW and was made well before there was any need for a wall or a nights watch.