I managed a community project a long time ago in Blitz3D...I started with a basic level...with player and collision and the basics for the game...
When participants see this, ideas start to flow to add to the game...trying to start just with ideas can cause a failure...
...Like Thirsty Panther said...beo6, I would recommend that you start the project and get a basic start to the game and then get ideas from the people who are interested in the game you started....I used this process back in the Blitz3D days and was successful in keeping people working on the project for over a year...The game was not finished, but was starting to look pretty good....
I'm trying to tell you what to do, just trying to help out with some life experience....