I think that you will find, that unless you wanting to create a simple game that has a closed ending, that things will always diverge from your original concepts
The thing to do, is to make sure that you are clear on where you want to go, and then rein yourself in where needed to maintain that initial vision.
For example, I know that I have stressed the idea of allowing for expansion (to that point of being able to handle tougher critters/foes), but thinking about that expansion and how it would affect things doesn't mean you need to change your initial ideas...
Plus there is also the fact, that once you actually start playtesting, things WILL diverge from that initial vision, as you find that some things work well, and others not at all how you imagined them to work. There is no way to avoid that. The idea is to think about alternatives, and how those alternatives will fit back into your initial vision, and to try and stay as close to that as you can.
Anwyn
