As of early 2015, the eighth book in the Safehold series, entitled "Hell's Foundations Quiver," is scheduled to be published September 8, 2015, but the book will likelycontinue to follow the exploits of Merlin and the country of Chalis as it rises to challenge the power of the Church. Selected snippets of the book are available on the author's website.
The series has so far followed Merlin, a remnant of the more advanced age of human civilization. His many powers and cybernetic body have so far been kept a secret from the inhabitants of Safehold.
In the eighth book, a new character is set to enter the fray and has detailed knowledge about Merlin, as well as his abilities.
Given the increasing tension between the Church and the nation of Chalis that Merlin has been influencing and helping to develop technology, it is likely the eighth book will focus on the growing conflict between the major powers.
The introduction of the mysterious new character could also reveal information about Merlin and his origins to the rest of the Safehold world. How they react to such information could be a major component of the ongoing story, especially given how influential Merlin has been in shaping events.
The true purpose of the Church, to keep industrial technology stunted, is also likely to be revealed to the various countries in the world.