My theory was that since GRRM always steals good storylines from other franchises I thought he would steal this one from Wow- Wrath of the Lich king.

When the LIch King dies, there must be someone to take his place. There must always be a Lich King, and in this scenario Night King.

He needs a host to take his place, to lead the undead army and that person would be Jon Snow.

Now that the undead army is dead, somehow I don't see it as a possiblity.