The inconveniently lame possibility that GoT is all about Bran and he's basically omnipresent, everlasting time warg and as such he can see what various rulers, including Denerys or the human wars will become, and therefore occasionally provides "(counter)balance to the force" warging into dead bodies and rising army of dead. It would be a terrible and cheap twist, but in the same time it fits the bill - Bran is already being presented to us as cold, distant, "I'll do what needs to be done" character, he's being kept alive while already irrelevant, yet absent from the first line of plots (classic sign that he will play an important part and writers want you to discard his character from memory for better "didn't expect that" effect) and then it explains why Night King always knows when Bran is following him and why NK didn't javelin the Magnificent 7 (clearly within his abilities) there and then and waited for better pay off etc.
I don't like it, but I can totally see it happening.