There is a subreddit called r/gameideas, and they have a few rules.
Did you wish to stipulate that, say, for example, the people creating these games get to keep control over it or anything? Specifically, they have rule 6:
6. Your Idea is Public Domain
Ideas shared on r/gameideas are free to be used by anybody. You may ask for credit if somebody decides to use your idea but they are NOT required to credit you.
Otherwise, I went ahead and subscribed to the thread! Should be exciting!