11. Euron's Role & Death
Euron Greyjoy was one of the biggest wastes of George R.R. Martin's source material, with the character effectively getting the name but few of the actions. His entrance, where he killed his brother and proclaimed himself "the storm", held a lot of promise, but after that he became just whatever the plot needed him to be, while swaggering around like a bad Jack Sparrow impersonator.
In the books, it's a VERY different story, where the Crow's Eye is steeped in mysticism. He drinks Shade of the Evening, has learned secrets from all over the Known World, sports Valyrian steel armour, has killed and molested his family, and basically combines the evil of Ramsay with the cunning of Tywin and the madness of Aerys.
As it stands, he's intent on either marrying Daenerys Targaryen (sending his brother Victarion to court her) or, at the very least, binding one of her dragons to his will using the horn dragonbinder, with the latter a much likelier outcome in the books. The show sort of hints at this too: not just in having him kill a dragon, but in how the Night King becomes the big, supernatural evil presence and takes a dragon, likely taking some of Euron's story in a fashion.
Euron himself is moving against the Reach, where he'll be going up against the forces of House Redwyne and Hightower. But Euron's plans are much, much bigger than simply wanting to get himself a Queen - be it Dany or Cersei (and it won't be Cersei) - with hints that he wants, in some way, to bring about a much more apocalyptic ending for Westeros, either to serve himself or some higher purpose (or make them one and the same). Euron seems unlikely to make it to the very end of the story, but he'll be a big old Lovecraftian nightmare while he is around, destroying the forces of House Hightower at the very least while bring more magical (and horror) elements into the story. He will probably never meet Jaime Lannister, let alone be killed by him in a fight on the shore.