In an interview posted by Game Informer, de Jonge disclosed that Guerrilla Games was working on prototypes for two new games that were considered going into development. The first one is obviously Horizon: Zero Dawn, while the other project was never revealed. However, it is highly likely that the unknown title was another Killzone sequel. Speaking with Game Informer, de Jonge revealed how they decided what game would make the cut:

It seems like Guerrilla Games had a bit of a competition going. They were unsure of which game would be best for pursing development, but eventually, Horizon: Zero Dawn was the most favored.

Everyone was clearly more interested in taking a different approach. Even Hermen Hulst, the managing director, commented on the decision to pursue Horizon: Zero Dawn by saying:

We’ll never know for sure what the other project was. Although these directors have both said that Killzone was “beautifully gritty,” they are more interested in taking a different approach by making something more “beautiful in a majestic way.” Fans will have to wait and see if this is true with Horizon: Zero Dawn.

Due to the lackluster performance of Killzone Shadow Fall, it was most likely a sequel to that game, but Guerrilla Games has seemingly moved on from that franchise and is now focusing on another one. Let’s hope that it lives up to the hype.