I thought this would be fun game to play since video game movies have a stigma for being bad and nobody has really figured out how to do them yet. The Tomb Raider one that came out recently is supposed to be okay from what I heard but nothing special, Assassin's Creed apparently they spend most of the film in modern day which is crazy when you have all sorts of historical time periods to visit instead. I wonder why they didn't just scrap that part of the game.
Anyway here's my idea:
I would adapt Shadow of the Colossus but have the guy only need to kill one Colossus (probably I'd pick the third Colossus or maybe the flying one in the desert) to save the girl instead of 16, and try to build the relationship between him and his horse throughout the film. He would have to speak more to the horse though so you can build the relationship whereas in the game he's fairly mute. Most of the film would be about that relationship with the horse and his travels across the Forbidden Land, scaling tall mountains, through ancient and abandoned cities (like the one where you fight XIV aka Cenobia) and other landscapes, or camping at nighttime feeding the horse. There wouldn't really be any other characters than those people that arrive at the end after he gets turned into a monster. At the Colossus battle I would try to depict it the same way as they do in the games, not some rampaging monster like in Pacific Rim or Rampage but as an animal that you could feel sympathy for since it's just minding it's own business and ends up being murdered. Maybe in the fight scene do what they did in Dark Souls where you can see it's injured and in pain towards the end like with the Sif boss battle when the wolf is damaged and you see it limping and in pain. I would possibly kill the horse toward the end but not during the battle itself, maybe just before like in the lead-up to Colossus XVI where the bridge collapses and it falls but in the game it survives, I would rather be more decisive and kill it instead.
I'm not a scriptwriter though it would probably be really boring but I think it would be cool and could actually work.