I think they'll do a lot of surgery on books four and five. The Mereen plot will probably be kept, but radically altered. The basic idea of Dany ruling a city and handling a terrorist campaign has a lot of potential, but needs to be much more dynamic for TV than it was in the book.
Also, they'll probably bring the climaxes that have been delayed until the sixth book forward into season six. So seasons five and six will basically cover Feast
and maybe the first quarter or so of Winds