You might've noticed that Guerilla Games' newest project Horizon: Zero Dawn has lush environments that look far more open than Killzone's linear corridors. That's because Horizon's design is more organic than anything the studio has worked on in the past, with wandering herds of robo-dinos that you can happen across during your hunt. Making that transition from the scripted to a more open and free-form design is difficult work, especially for a studio that doesn't have a whole lot of experience in that area. That's why Guerilla specifically sought out designers from famous open-world franchises like Fallout and The Witcher to help create its new game.
According to an interview with studio art director Jan Bart Van Beek and lead producer Lambert Muller, the biggest challenge for the team was the shift from making "a level that needs to take 45 minutes to an hour, to making an experience that needs to go into the dozens of hours, that maybe has dozens of things to do." So the best solution for Guerilla was to hire talent that knew how to make these sorts of games already. The studio brought on developers from Bethesda (the lead writer for Horizon worked on Fallout: New Vegas, for instance) and CD Projekt Red (the studio behind the massive fantasy RPG series, The Witcher).
These are developers who know how open-worlds work, and were brought on to help Horizon grow and expand, to turn it into a "machine that works, where every moment is fun no matter how you approach it." Guerilla is essentially learning how to build a sprawling game like Horizon from scratch, and while their ambitions were initially incredibly high, these devs stepped in to help them realize why certain elements of open-world games work the way they do. "This is a lesson learned on how to [avoid] the same mistakes that everyone's going to make when they first [develop] an open-world game," says Van Beek.
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