I think the main problem with the world of Horizon is that the most interesting event in their world has already happened.

The story of Zero Dawn worked so well because it is the interwoven tale of a young woman who sets out to discover why she was cast out of her village at birth, and the almost archaeological unraveling of why the world is the way it is. When you finally piece together both the plot is almost already at it's climax, and you are left with both the understanding of why it must be Aloy who stops the new threat to the world, and the motivation to do so.

But that doesn't work for a sequel. The format of Zero Dawn relies on exposition about the very nature of the world, that's why the main quest has a bunch of missions that more or less boil down to walking around an old facility and listening to recordings.

How are you going to translate that into a new sequel? Either you've got sequels planned already, which I find unlikely given what Forbidden West amounted to, or you need to try to invent more world building and plot. It seemed quite clear to me that Guerillas writers for Forbidden West didn't know their own world as well as I had assumed they did. The "how did we get here" plot in Zero Dawn revolved around a small cast characters, who, with the exception of one, were all both very neuanced and strongly invested in their own plot. The Zeniths of Forbidden West come across almost as inverse Deus Ex Machina, characters who fly in from the moon with what seems like no other reason to mess up the plot than "We had to find something".