I think making Bran a hidden antagonist is an interesting idea, because you can use some clever editing to imply that he manipulated events to put himself in power.
Instead of making Bran the Night King, though, I think it would be interesting to give more characterization to who, or what, the “Three-Eyed Raven” actually is, and make his motivation clearer.
As we learn in the show, the Children of the Forest created the Night King and the White Walkers in order to stop humanity, who were encroaching on their lands and killing them. But, the Children lost control of their creation, and made a pact with the First Men in order to stop them together. But stopping the White Walkers did not their problems with humanity, since the Children basically went extinct in the end.
But how does the Three-Eyed Raven fit into the picture? What is it, exactly? What if the weirwood trees actually had as some sort of consciousness (the old gods), and the Three-Eyed Raven was merely a vessel for this entity?
In the Night King creation flashback, there is a quick shot of the face of the weirwood tree there. Maybe it could be edited in a way to imply that the weirwood is speaking to his Children (giving the name “Children of the Forest” have extra meaning). The weirwood could be the one to tell them to create the Night King. You could even edit the weirwood face to have three eyes, to help hammer the connection to the weirwood and the Three-Eyed Raven.
It also would be a good explanation for why the Night King personally wants to kill Bran and why he doesn’t go over the wall until after Bran has already gone South. The Night King wants to kill its creator.
You could even imply through montage visions that the Three-Eyed Raven was behind many of the “miracles” credited to the other gods. Basically, it would make nature itself a player of the game of thrones, playing the long game to fulfill its own mysterious goals. We could see how it moved the pieces throughout the show to take over Westeros, like it once did before men ever set foot there.
This is not a bad idea. Not bad at all. The problem with it is that there is so little material to work with to make it a satisfactory cliffhanger. We are hardly introduced to the Chikdren. At least the night king has a presence.