I've always felt the biggest mistake with Killzone was putting us in the perspective of uninteresting ISA characters. The most iconic aspect of Killzone are the Helghast. We should have taken charge of some badass uber ninja Helghast soldier.
You know what gamers like? Anti-heroes. You know what always go over well with audiences? Redemption stories. Mix the two and there's your new protagonist for the new trilogy. A rightfully jaded Helghast elite soldier who begins to question the war and the reason for fighting on both sides. Being a ninja elite, he can have a few more abilities than just shooting and stabbing. Imagine those ninjas from KZ3, but upgraded.
There's bound to be more Killzone on PS3. Seriously, GG. Read this and take note. Change it up and leave Sev, Rico and the like in the past.
PS4 is like a fresh, new chance to blow minds and get a larger audience on board for your series. If you do it right, and you're there for launch, it could basically be a re-birth for the series. Do not continue with such boring protagonists. You need someone memorable at the helm. You already have a memorable design. Just jack the age old tale of a fighter changing sides or learning to fight for something different. Use the lore of the series. Make it interesting. There's potential there to make it quite big.