It seems that there won't be any new games for this franchise in the foreseeable future.
If you're a loyal player of the long-time franchise Killzone and would check its official website from time to time, there's chance that you might have found out the site is no longer running anymore, or as described by its developer Guerrilla Games, "The official website for the Killzone franchise has retired." Soon, visitors of Killzone.com will just be directed to PlayStation.com.
In a statement, Guerrilla Games also mentioned a few possible impacts: "While this change doesn't affect the online multiplayer modes, player statistics or ranking data for Killzone Mercenary and Killzone Shadow Fall, it is now no longer possible to create or manage clans in Killzone Shadow Fall. We apologize for this inconvenience."
It is unlikely Guerrilla will have time to deliver any new game for Killzone anyway, since they should be really busy with Horizon Forbidden West, the highly expected sequel for 2017's Horizon Zero Dawn now.