Guerrilla Games are moving to a new building in order to expand their offices at some point during the start of 2019. Horizon Zero Dawn's developers stated that this move is happening due to their wish to speed up their game making process.
According to Guerrilla's CEO, Hermen Hulst, the studio has been looking to expand for over three years now. It currently has about 250 employees but the management decided it would best if they could increase the capacity.
The studio has announced they will hire about 150 additional people, for a total size of 400 permanent employees. Hulst has stated that they have already hired nine people in an interview with Het Parool.
Hulst also hopes that the increase in the number of employees will cut the development time for the studio's future games, citing Horizon Zero Dawn's development that took over six years as a thing he would like to avoid in the future. He is hoping that new and current IPs owned by Guerrilla will have two or three year development cycle instead.
This would also mean potential new entries in the Killzone franchise could happen in the future, instead of staying in the shadow of Horizon Zero Dawn's success. Killzone: Shadow Fall was the last entry - released in 2013, five years ago.
Hulst's previous statement about Horizon Zero Dawn's development period confirmed that they were being developed at the same time initially and his later statement about broadening the scope of development hints at the possibility of developing sequels for both games simultaneously.
This is all just speculation at this point as nothing has been confirmed yet, but it's hard to believe Guerrilla Games will just step back from making a success for Horizon Zero Dawn. That said, a Killzone entry may still not come to pass if a new IP indeed happens and grabs the developers attention instead.