Guerrilla Games is known for the Killzone franchise, and that isn't changing any time soon. During an art process panel at GDC 2012, Guerrilla's Studio Art Director Jan-Bart van Beek said that the team -- which now includes the Sony Cambridge Studios working on Killzone Vita -- is simultaneously working on three different projects. There's the aforementioned Vita game, a new non-Killzone title, and continuing the franchise on the PlayStation 3.
Jan-Bart van Beek didn't say if the PS3 stuff was supporting Killzone 3 with downloadable content or a new entry in the franchise, but he did talk about how the team decided on its new IP. Basically, interested team members pitched the next game they'd want to make -- 32 entries to start with -- and the group narrowed it down to four.
Now, Jan-Bart van Beek didn't say if these were the final four types of games, but he did list zombies, werewolves, steampunk and Ghost in the Shell as pitches. Apparently the steampunk one didn't win out, but it did get iterated on. Eventually the team morphed the idea from being a Steampunk Sherlock Holmes taking on monsters like Frankenstein to a retro-futuristic '70s-punk title called Dark Science. Players would be Jake Crowbar, a Dirty Harry type, who takes on all sorts of monsters.
Again, Dark Science didn't pan out and Jan-Bart van Beek openly stated it won't be Guerrilla's next game, but it was interesting to see the pitch's path.
IGN - Three Projects from Killzone Devs in the Works