It'll be a different story for the most part by default. I'm convinced that Barristan, Victarion, and Stannis will die in the first third of TWOW, so that alone changes things a lot. Without Aegon, I feel the story changes drastically in the show. We all know Tommen will die but we don't know when and how? Those two will certainly differ from book to show. Jon's resurrection is going to differ as well since it's pretty much guaranteed in the books that Melisandre will sacrifice Shireen to revive him.I hope they've seen little to nothing of Winds
The less bastardized version of the real story the better. Let them sink on their own
The only things I can see the show spoiling the books on now are:
- Jaime killing Cersei (if that is indeed how she goes out)
- A surprise death for Arya, Tyrion, or Littlefinger. Sansa's plotline differs so much that even if she dies at some point in the near future we can be certain that things will be different in the books. And Davos might be absorbing some of Stannis' and Asha's material.
Basically anybody aside from Dany, Jon, and Bran has a good chance of dying before the end. I would like to add Littlefinger and Varys to the group but who knows what Martin has planned.
As for the death GRRM finally figured out, it's 100% not Cersei. If he wasn't sure how he's going to kill her, he wouldn't have put the Valonqar prophecy in the story as well as the foreshadowing for Jaime being the Valonqar.