From what I see people die when their character hasn't shifted in a while or their arc is coming to a close and if they're not going to change they're going to die.

1) Grey Worm (Any day now)
2) High Sparrow (Late Season)
3) Brienne (Furthering Podrick's arc to protect Sansa himself)
4) Ramsey (End season)
5) Loras Tyrell (Which is what kicks the High Sparrow's death into place)