They are focusing on something else (which is a good thing).
The Netherlands-based team already had a massive title out this year in the shape of Horizon Zero Dawn, which will get its first DLC, The Frozen Wilds, on November 7. What about Killzone? Hermen Hulst, Guerrilla‘s managing director had an interview with the Academy of Interactive Arts and Science as a warming up to the D.I.C.E. Europe event. He mentioned Killzone, too.
„I get asked that question [about Killzone] a lot through Twitter and other social media channels. It’s heartwarming to hear that there’s such a strong appetite from the loyal Killzone fanbase. It’s a franchise and universe that we still love with a passion at Guerrilla. It hasn’t been easy doing something so completely different. Whether or not we’ve put a permanent period behind the series, it’s too early to make announcements on that. All I can say is that we’ve always loved the Killzone universe and the Killzone series, and we still do. We share that passion for the game with a lot of our fans.”
What he means by this comment is that if there will be enough demand, they will start working on the next one… (Killzone Shadow Fall was a launch title for the PlayStation 4 almost four years ago!)