The idea of certain tools being available depending on your music seems a little restricting to me. I'm not sure if you wrote down the same thing that's in your head.
About a God game though - I had the same idea. But instead of creating the actual world with music, or with musically oriented tools, I thought you could just be a demi-god tasked with introducing music to the world after the major Gods have shaped it and populated it. Your would become a sort of meistro of humanity, by dropping music into the world that would affect people's actions and feelings. Like, playing some heroic music after a small kingdom was destroyed could lead the people to rebuild, or playing "nature-y" music could revitalize the surrounding natural forces and cause plants and animals to retake the region. And there would be key heroic figures that spawned in random places throughout the land whose fate's you could guide by playing music to motivate them to puritan heroism or ambitious cunning. Something like that... putting music into a world to guide it's path. I definitely don't have the details.