Guerrilla Games has confirmed that they are not only working on Killzone but on another mysterious ip. Coming from a interview from Eurogamer at EGX 2013 Lead Designer at Guerrilla Games mentioned
“As a studio we do want to branch out, and we have started work on a new IP, something completely different to Killzone. I don’t want to say anything about it right now, but as a studio we do want to keep it fresh.”
No system was mentioned, but does give you a bit info that that more variety is coming from Guerrilla Games for the PS Vita or Playstation 4.