I don't think that open-ended games are out of reach for movies. The movies made don't always follow the game exactly just like comics-made-into-movies don't always follow the storyline exactly. They take the general idea, the characters basic abilities, the more memorable villains and make a new storyline that would fit into the comic/game.
The same could be done with KotOR or BG if they really wanted to. But I'm not all that impressed with Lucas lately so I would rather they didn't make a KotOR movie. Unless Chris Nolan made it.

And Peter Jackson is making Halo so maybe that will be worthwhile.