I believe anyone hoping for something other than a team up against the white walkers during the last season are going to be disappointed. GRRM set out to deconstruct the fantasy tropes introduced by Tolkein, one example being speaking out against Gandalf's resurrection having no ramifications to the character and the good vs evil motif.
D&D have already brought back Jon with no ramifications, so far. They've also made it clear the white walkers are evil - describing the night king as an embodiment of death in an interview.
In some ways, he's just death, coming for everyone in the story, coming for all of us. In some ways, it's appropriate he doesn't speak. What's death going to say? Anything would diminish him. He's just a force of destruction. I don't think we've ever been tempted to write dialogue for the Night King. Anything he said would be anticlimactic.
http://www.telegraph.co.uk/tv/2016/...on-7-will-the-night-king-ride-the-winds-of-w/
Subversion of tropes and truly shocking events, barring hodor (which we already know is a GRRM plot point) alongside Shireen's burning (which i hope to god is handled a lot differently) ceased at the conclusion of season 4.
The show is on a simple path to the end now. Theory discussion between episodes have become more complex than the show itself. I remember last season when everyone was discussing Arya's plan to defeat the waif and how she wouldn't simply parade in public inviting the attack. It tuned out she was in fact parading in public, inviting an attack.
Though a spy leaking information to Cersei would be interesting, I doubt it will be the case. Euron is a magic deus ex machina machine with teleporting ships brought into the show to level the playing field against Dany. I hope we do see him betray cersei at a point though, he was originally planning to take the Iron throne through force.
Edit - One twist I'm betting on happening is Jon Snow actually dying once the white walkers are defeated as the lord of light no longer needs him.