"I anticipate a dark reunion for Arya and Melisandre, one that may prevent Arya from ever finding her way home. We will witness Jon and Dany’s realization of their true relationship, and find out if Littlefinger has any meaningful leverage left now that Sansa has seen him for what he really is. But I'm most intrigued by what Sam will learn at the Citadel. The Faceless Men stole something of value from within during the 'Alchemist' chapter of A Feast For Crows, and whatever it was might hold the key to defeating the White Walkers. Judging from what we've heard through Arya's training in the House of Black and White, it isn't a surety that the Faceless Men are on the side of the living.
"I do not, however, think we will see a resolution between all the competing eschatologies of A Song of Ice and Fire/Game of Thrones until the final season. Characters that could determine the fate of the world variously follow The Seven, The Lord of Light, The Children, the Drowned God, and the Faceless Men, not to mention the mysterious magic of the Dragons and the White Walkers. Euron's quest to subdue and enslave avatars of all the religions of the world may give us our window into which are supreme. I also suspect that next season a main character will be absorbed into the White Walkers' army and give us our first insight into their motivations, if they even have motivations.