“theories about the Night King”
He is a Stark, and there is an empty tomb waiting for him in the crypts of Winterfell.
Samwell Tarley breaks out the magic Horn of Winter that he has been lugging around since he found it at the Fist of the First Men. Blowing it resurrects all the Kings of Winter from the Crypts - possibly even including the dead Night King. After the crisis is past, they all return to their graves, including the Night King. Jon or Bran then replaces him. Blowing the horn to start the climatic battle would make Sam the equivalent of Heimdallr (who kills Loki) in the Ragnarok theory.
Alternatively, Samwell Tarley finds a cure for the cursed Night King in the books he brought from the Citadel (green Dragonglass), that restores his human form, and allows him to find rest in his rightful tomb in the crypts.
Samwell looked like he is having a hard time with Daenerys killing his father and brother (more with his brother). I figured he wants to confront the dragon queen at some point and it wouldn’t end well for him. Or will he just be a wedge between Jon and Daenerys? Hmm...
That’s more likely in the books than the TV show.