If anybody at HBO has any brains they have to know that the final season effectively killed one of the single most profitable I.P.s in current production. That's a massive loss, at that point, do you stand to gain more from doing a hasty sequel that 'fixes' the finale -- damn the budget -- or not? I'd say, it'd probably be worth it in the long run for them, especially with twenty other spinoffs in the can and George twiddling his thumbs and making a fat ass of himself on social media.
There might not be much to work with, but a hasty walk-back wouldn't necessarily go bad with the fans. Basically make it a proper Long Night. The NK didn't really die, more zombie stuff, just sort of repeat all that squandered Winter is Coming shit and you could maybe get some people back on board. It's risky, but when your golden goose is sick and dying, isn't it worth forking over thousands in the hopes of getting some more solid-gold eggs?
Bran gets visions of a real long night and sends an order to release Jon of his vows and come to King's Landing to prepare for THE REAL LONG NIGHT. Pretty much fanfiction tier but it can't be worse than anything else they make at this point so fuck it I guess. If they wanted, they could basically make it Game of Thrones : Apocalypse and tap into a little of that Walking Dead action.
Zombies showing up, ice spiders, the whole of Westeros got completely fucked in the war so there's little food and resources to prepare, turning people against themselves. It's all there, it's not very interesting, but it's there.