Platform for finding co-authors? (perma-thread)

A reader writes in:

I have had a raw idea for a few years that I don't know how to evaluate and am sure that I cannot put it into practice by myself, even if it has any positive points. Recently I saw a relevant post on your blog exactly about this. I thought I can ask you as you are in a good position to advise.

It starts with an (I guess common) personal experience. I argued a subfield of political philosophy, where I had no experience with the literature. For a while, I was thinking about how I could find a co-author - it would be much faster to develop such an idea with a co-author, plus it would be good to have real cooperation between subfields, fields, etc. But then it came to my mind that it may be a problem for many others too. So, why not have a platform just for that purpose?