Your technology is in place, your content creation tools are in place. Your artists understand the look and feel of the world. Your designers know what their maps have to accomplish in the context of the game fiction as well as what game systems they can use to deliver the gameplay experience players want. It's all, again, a gut-level sort of thing. But you're right when you say that projects develop all the time.
Game development is an organic process.
You HAVE to be open to change during production. The best pre-production process won't make a game great. Greatness only comes during the hands-on process of production. I suspect that some games require less and I can't really imagine taking much more than that. The DX team did a comprehensive post-mortem of DX1 after we shipped.