First of all, Horizon: Zero Dawn sequel was hinted at by Janina Gavinkar, Star Wars Battlefront II story campaign's protagonist, at the Star Wars Celebration.
That's all that the actor let slip though - it doesn't necessarily mean the game is anywhere near being launched, especially considering Sony is skipping E3 this year.
Gavankar voiced Tatai in Horizon: Zero Dawn's Frozen Wilds DLC - not a huge role, but big enough (and left open-ended enough) that it wouldn't be surprising for her to reappear in a sequel. "Thank you for getting me into 'Horizon Zero Dawn, '" the fan says in the video. Wait until you see the sequel.
Chances are you've still got a while to complete the original though. The Twitch clip has since been taken down, but it's rare that things on the Internet are truly gone forever; a mirror has surfaced on a Reddit /r/Games submission discussing the apparent leak.
For now, all we know about this sequel is that it exists in some stage, so questions still remain on when the game will come out, and what it will come out on.
However, there aren't any other details available on the Horizon Zero Dawn 2 as of now.