Either Cambridge is making Killzone, or Killzone is dead.Guerrilla Games confirmed around E3 or not long after that they aren't done with Killzone.
They were repurposed into a sister studio for Guerilla just to take over the franchise. Vita game flopped sales wise but was still an awesome game. Had a better meta than Shadow Fall. Plus since Resistance is dead, Killzone is Sony's only FPS. Definitely deserves at least one more entry.
EDIT: Guerrilla Games Isn't Done with Killzone Just Yet - Push Square
Sounds like they're either giving it a rest for now or don't want to say too much.