When I was at the last GDC, there were some good points in Satoru Iwata’s keynote about how they develop game ideas at Nintendo:
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First Miyamoto gets an idea for a game by observing things that people find fun, and analyzes them to find out what is fun about them. Quite many of his ideas come from his hobbies (gardening produced Pikmin, getting a pet produced Nintendogs and even Wii Fit came from his hobbies), so Iwata said that he has made Miyamoto sign an NDA for not talking about his hobbies outside work ;D.
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Then when there is an idea, a small team of maybe just one developer will work for months or years on building a prototype game which concentrates on what makes the game fun. At the same time, multiple prototypes are in development on their own pace (Iwata will never ask how a prototype is progressing, for fear of giving pressure to finish it more quickly). Only after the prototype is fun, the production of a publishable game begins together with a big development team. Then they know that the core concept has been proved to be fun, so they always have something to which to return if the actual game is not anymore fun.
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During development, hallway usability testing is used much. Miyamoto kidnaps a random Nintendo employee (non-developer) and forces him to play one of the games, while Miyamoto silently sits by and watches what the player does - what is fun and what is frustrating.
I think that by following these ideas, especially the point about first creating a fun prototype, would help a community project like this to succeed.