People can throw ideas here and there, but eventually every person has to do a specific job. Or team up with a few people. So the best way to go is to built a team from the beginning and giving specific tasks to everyone. Otherwise it just is too vague.
You should at least decide what kind of RPG. An online one? An RPG which has a beginning and an end? You want an RPG that people can play through facebook? A one that you can play with random people using a smart phone? An action/RPG type? A FF tactics kind of style? An RPG for linux?
Specify more the platform and the type and have an idea why would people bother to play it (innovative story, unique period, rare platform etc etc) and not play a commercial one or the many free ones...
