IMO, I think a movie set in the end of the world of the ancients, when the Zero Dawn project is created, could be way more interesting than an adaptation of the first game.
I reckon it would also be a lot more expensive due to being a lot of more action, but there would also be a good amount of intriguing exposition (Ted Faro, Elizabeth, the military heroes, moral and emotional dilemmas of Zero Dawn). A bittersweet ending at the end of the world but with the hope of what comes after. I guess that would mean if there were to be a sequel (set in current Horizon time-line), they can't rely too much on what happened in the end of the world for exposition and intrigue.
Also, they could skew the story somewhat even more towards the AI aspect. The game already does it a good deal, but not in a way that is directly relatable to our current situation with AI. If they focus on that (potential and risks of AI, human input on AI, corporate greed, etc), and they market it as such, maybe it could be a very successful movie. I would definitely watch that, at least.