Before Guerrilla Games brought us Horizon Zero Dawn, they brought us the Killzone franchise. The first Killzone release in 2004, it's sequel released in 2009, Killzone 3 in 2011, and Killzone: Shadow Fall in 2013. Since 2013, Guerilla Games hasn't mentioned much about Killzone, despite being asked about it.
Guerrilla Games’ managing director, Hermen Hulst revealed that he gets questioned on the stated of the Killzone franchise quite a bit on social media during an interview with the Academy of Interactive Arts & Sciences.
Hulst couldn't specify whether the franchise had been thoroughly put on the back burner, but he did say he appreciated the interest from fans.
“It’s really 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.”
There's no definitive answer on the future of Killzone, which probably isn't a response that Killzone fans want to hear.