I think the real twist is that fire will not work on the NK.
The NK wasn't afraid of the dragons. Walked through the fire created by the COF, and that twist was set-up by Jon's question to Bran last night.
The twist in the battle will be a moment of what feels like triumph when they take down the undead dragon (looking like they're about to finish the NK) but when they're about to deliver the final blow, he resists it and instead delivers the killing blow to Theon or another character of importance.
I also would love it if the NK caught Longclaw (a play on the Longclaw twist in Hardhome in S5), and shattered it when it seemed like Jon was going to deliver the killing blow.
In short - I will be very disappointed if the White Walkers are taken out next week. I think the seeds have been planted for KL burning for a while, and I'd love if the Walkers were the cause of that.
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