Honestly I think the game would be better if it was just based on the world and not the actual book storyline. I have read every book at least five times each and there isn't really that much action throughout any of the books.
The story just wouldn't fit well into a game. Once everyone split up it would have to jump between characters a lot, so the game progression would feel fractured. This would also limit any Rpg elements because it would basically be leveling multiple characters separately which would start to feel tedious the farther you got into the game.
If they made it where you had to pick one of the characters at the beginning of the game then it would be like making a storyline for each of the characters causing more work for the developers. Which in turn would make the game more expensive to make so they would have to cut things out or make it shorter then it should be.