I've been wondering the same thing... I think it might be a good subforum to have for a number of reasons. The top two reasons on my list are: first, to provide somewhere for those that do not know what mysticism is to gain understanding and second, to provide an area for mystics to discuss and compare their experiences, ideas, and so forth. There seem to be a lot of people that I meet in the U.S. that think mysticism is a recent new age phenomenon or that it has something to do with magic. This might help give a place for folks to go to that wish to know what it really is and the long history of it in the Western religions. Secondly, I can say that as a mystic, I find a lot of resonance with mystical writings from both within and outside of my religious tradition. I think it'd be interesting to have a place to discuss such writings as well as our own experiences and ideas across religious boundaries.
Maybe it would work as a subforum to B&S, or I suppose we could go with only Western religious mysticism and put it as a subforum on Abrahamic. Personally, I prefer to be as broad and inclusive as possible, but anything would be nifty.