There's still book material to adapt
While a few key characters have indeed caught up to or surpassed where their storylines ended in the books -- primarily Daenerys, Jon Snow, Tyrion, and Cersei -- the show's most recent season took major detours away from the plot of the books to cover more exotic locales and bring characters together. Now they're backtracking.
After the fan disappointment that was Dorne last year, look for all eyes to be on the quieter, less gorgeous Riverlands, a key location in GRRM's fourth book, A Feast for Crows. That corner of Westeros will see the likes of Jaime, the Tullys, the Freys, and a new iteration of the Brotherhood Without Banners -- all staples of the show's earlier seasons that fell by the wayside after the Red Wedding.