I recently asked a question about a theory proposed by someone I know. It is not a scientifically known one, but it is not so obviously true or false that one can avoid speculating on it, to reason out why it should be false.
I admit, it was not a "mainstream" question as required by the site. My question got closed with the reason that it was not mainstream, and theories can be discussed only if they have already been researched, new work is not allowed on the site.
However, before its closing, there was a person who commented that they could discuss it according to the known facts. There was 1 upvote on it. Someone else countered him by saying that the same could be done with any off-topic question, hence that was not a valid reason.
Anyway, it shows that there are people willing to answer it.
Would it be a good idea to create an "off-stream" tag, where unpublished theories can be discussed scientifically. Those who don't like the idea could just avoid browsing through such questions, instead of banning them from the site altogether.