Game mechanics will come later. Here's the general game development process we'll be following:

1) Brainstorming (current step)
This is where general story and character ideas are developed until we think we have something that might actually work.
2) Original Treatment
We compile a list of all of the desired features (practical or not). This is where we get to have fun and not worry about the limitations of technology or our own abilities.
3) Game Specification
We create a more feasible list of features, storyline elements, etc. This time, we pay attention to limitations.
4) Design Document
This is a series of text files, images, etc. that give an in-depth description of how the game will work. It will contain every detail of every aspect of the storyline and gameplay.
5) Artwork
Creation of concept sketches will begin at this point, and this will move on into produciton art as the project continues.
6) Testbed Development
This actually happens along with step 7. It is where we create several simple script/programs to test our design and programming theories. We work out bugs with our design here, before we've spent hundreds of hours coding.
7) Game Architecture
This is where we disect the game and define exactly how it should be programmed. We define the specific technologies, libraries, class models, etc. to be used.
8) Development
Type, type, type. Compile, compile, compile. Test, test, test.

That's a general overview. There are several steps between and after those mentioned, but you get the idea.

-Nag