Mmm... the Kings Landing theory is nice, especially with only 4 episodes to go. Next ep could be KL and Cersei getting her just deserts while the North waits, looking puzzled. Saves them having to rush south and sort that loose end out later. But I'm not sure what the NK would get by toppling Cersei other than a new southern army and a pincer movement on the North that seems unlikely to be required. Can he resurrect dragon skulls? Seems unlikely. Cersei'd make a cracking Night Queen, but I don't see what she'd add that he needs.
I realised while watching this episode that I've already made my peace with this series. Endings rarely scratch the itch properly in any story, and they've earned the right to get it a bit wrong. So if this all gets a bit 'Line of Duty' (I though last night's LoD was closer to farce than drama) I can live with that. I do struggle to work out what the 3-Eyed Raven's for though. Ok he/it's a kind of 'memory library' for humanity, but what use is storage if its not used for something other than making Jaime squirm? What did the 3ER do in the past except sit under a tree in various incarnations. Or have I missed something and he was instrumental in keeping Winter at bay or something similarly unsung-heroey? That would make sense, I suppose.
Oh, and I also like the Arya for NK or at least dragon slaying duty theory. I'll eat my hat if she survives though, much though I'd like a spin-off series following her round Westeros as a vigilante for the poor and downtrodden, with The Hound as her sidekick.
Final crackpot theory: The Mountain kills the Night King; what's already dead, cannot die! But we know The Hound has to snuff his brother for poetic justice, so maybe not. Whatever happens though, I reckon it'll keep everyone arguing even longer than the Sopranos' ending.