This seems to be a niche that nothing else fills right now. It's much larger scale than tabletop D&D. Much more freeform and literary than MMOs that do have a large scale. And asynchronous except for the monthly deadline.

Unfortunately it also feels like, to do it right would require it to charge significant fees. There would be a lot of effort required reading, collating, adjudicating and responding to each and every player even if groups doing roughly the same thing got shared replies. Probably more than an individual game-runner with a hobby could do.