Some individual studios host similar forums at the moment, where they encourage feature requests from their fanbase.
In house game testers, and public beta testers also provide a similar function, suggesting ideas and improvements to a game in development.
This is usually for a specific game, rather than a genre, which you are talking about.
Some things to think about… (just the first couple of things that popped into my head).
If the site is to be any way useful or successful, you would need a way of pushing the better content to the top to make it easily viewable.
I’d be fairly sure that if you don’t have some sort of a ranking/rating mechanism, the site would become useless fast, as it fills with detailed rubbish ideas from a minority of the posters, making it impractical as a resource. (Think Youtube or any web2.0 site without a rating system – bad enough with one!)
You might also want to think about putting some thought into imposing a firm structure on user submissions? (Force them to provide a good bit of metadata about their ‘idea’ or piece of content or whatever, rather than just allow it to be in an unstructured post).
If you just go with standard web discussion forums like this one, but filled with game ideas and argument about them, it would probably become useless fast..?
You would have to decide and enforce a firm policy on intellectual property – presumably that everything posted publicly on the site was public domain or something similar.
If you don’t solve this issue somehow, then no commercial developer will even visit your site for fear of opening themselves to future litigation.