Don't forget, he was brought back "for a reason." I wouldn't be surprised if after killing the night king he starts glowing and evaporating while everyone gets all teary-eyed and he's just like "it's alright, this was my task in life, with it completed I must return to the lord of light's side." *fades away into nothingness, dany hugs her baby manlet close and cries*
How much more cliche could it get?