“If a new magical resolution appears now, it needs to have been well-foreshadowed in the already established lore of the saga.”
Maybe the Horn of Winter? I think that Sam still has it from the Fist of the First Men. His ancestral home is even named Hornhill, which might foreshadow his role, in sounding or fixing the horn.
Another one is Sam healing the Night King, maybe with some green dragonglass.
Sam is in the crypt.