Horizon Zero Dawn is almost definitely going to get a sequel, and probably more beyond that. Sony Interactive Entertainment America president Shawn Layden confirmed that the future belongs to the robosaurs in an interview with The Telegraph: we don't have any more details yet, but you can expect more from Guerrilla Games' beautiful post-post-apocalyptic world.
“The reception for the game has been fantastic," Layden said. "[Guerrilla Games managing director] Hermen Hulst has got a very keen mind on where he wants to take Horizon and what the roadmap is - and that roadmap is expressed in multiple years. I think we’ll be in the Horizon business for a long time.”
Horizon Zero Dawn's "The Frozen Wilds" DLC will be the next chapter when it arrives sometime in 2017. But if the Horizon roadmap goes for "multiple years" at least one more full game is a given. Earlier in the interview, Layden observed that Sony and its studios have done a lot of "heavy lifting" setting up unique franchises and they don't plan on letting them lie fallow.
“That’s something else you’re going to see from us coming out over the years: how can we better leverage all these franchise IPs that we have?" Layden added.