Here's my theory that would completely kick ass:
Bran and Meera make it out far enough to see some wildlings and a pet giant making their way towards the wall (after hearing the wall is a safe spot now for wildlings). Bran fucking wargs into the giant like a boss and destroys the white walkers.
It could just be more wildlings or children of the forest that save them too. When Jorgen was killed the situation seemed hopeless as well, so there are plenty of options here.