I think it really depends upon the which topic the idea is. You need to find a group of people with knowledge (and interest) of the topic. A subreddit about tha

I think it really depends upon the which topic the idea is. You need to find a group of people with knowledge (and interest) of the topic. A subreddit about that topic seems a good fit to me, another option could be to look for a forum discussing that topic. If your problem is that you would like to not spread your discussions amog many different platform, I'd guess stay on Reddit and look for the most fitting subreddit.