Well of course it is going to happen with such successful game as Horizon Zero Dawn is and here is some info. Spoiler... not much
Aloy is not going anywhere, I am sure of that since she is so great character and players love her. Same as robo-saurs, who are basically main content of the game. Which leads me to believe the second game will be a sequal in possibly far away place from the first one with new robots and new history of mankind.
We are pretty sure the Horizon Zero Dawn 2 will be exclusive to Playstation platform. But is that going to be PS 4 or PS 5? Or both? We think it'll be PS 5 since the first game had such a beautiful graphics and developers will sure take advantage of new hardware to make it even more...
There are no official information about the game at the moment, just a post from Guerilla games they are hiring devs to work on the game. But it's in public space now and we soon expect official trailer and release date of Horizon Zero Dawn 2.
We'll keep you posted!